Altering The Timestamp In Perseus IQ .WAV Files

Three or four years ago I got into SDR receivers. I don't own a Perseus, but I do own and use an Airspy HF+ Discovery, an SDRPlay RSP1a, and an SDRPlay RSPdx.

Perseus IQ .WAV files can be found on the internet or compatible ones can be created by several SDR receiver programs. One of those programs is the HDSDR program. Even if you don't own a Perseus receiver (the hardware), you can use the free Perseus demo (version 5) to play the .WAV files.

Perseus V5 demo from MicroTelecom

These IQ files are fairly typical .WAV riff files. Within the file header is a special field which specifies the date/time of the recording. The Perseus receiver software picks this up and uses it in its display of the recording.

Occasionally someone has the need to change the time of the recording, possibly because their computer clock was off when the recording was made. This can be easily done if you don't mind some byte twiddling in a hex editor.

Altering The Timestamp (2 ways)

Both methods require a manual procedure, one easy and one a little more detailed. I've tested both and they work.

The starting date/time timestamp field is contained within the Perseus .WAV file header. It's found at hexadecimal location x34 and is 4 bytes long. It is the Epoch time for the start date/time. Epoch time is the number of seconds since January 1, 1970 at midnight AM: 00:00:00 GMT. No ending timestamp seems to be used, rather they use the actual length of the audio file to calculate the end time.

We must change this 4 byte starting timestamp field at location x34. That would be bytes 34, 35, 36, and 37.

There are two methods to change this timestamp. Follow along, looking at the screenshot below.

Method 1, the easier method. You need:

1. The freeware hex editor program HxD. HxD is a nice freeware one for Windows that I use.

https://mh-nexus.de/en/hxd/

Start your hex editor HxD and open the Perseus .WAV file. With your mouse, select the 4 bytes starting at hex location x34 (34,35,36,37). In the lower right panel look for the item named "time_t (32 bit)". You will see that HxD has converted the 4 byte value at hex x34 and displayed the date/time conversion in plain text right there.

Each of the items in the right panel can be modified. Here's how to modify the date/time item:

Only after selecting the 4 bytes starting at hex location x34, make your changes to the date/time in the "time_t (32 bit)" item, then press ENTER. HxD will calculate the 32 bit Epoch time value and replace the one at location x34.

Save the file. HxD will also save a backup of the old file for backup purposes.

You are done except for renaming the file, see below after Method 2.

Method 2, the harder geek method. You need:

1. The freeware hex editor program HxD. HxD is a nice freeware one for Windows that I use.

https://mh-nexus.de/en/hxd/

2. Access to an Epoch timestamp converter (available online).

https://www.epochconverter.com

3. Access to a decimal to hexadecimal converter or a programmer's decimal-hexadecimal calculator.

https://www.rapidtables.com/convert/number/decimal-to-hex.html

Start your hex editor HxD and open the Perseus .WAV file. With your mouse, select the 4 bytes starting at hex location x34 (34,35,36,37). In the lower right panel look for the item named "time_t (32 bit)". You will see that HxD has converted the 4 byte value at hex x34 and displayed the date/time conversion in plain text right there.

Write down your new start date and time. Go to the Epoch timestamp converter site (or another if you prefer) and enter the date/time you have chosen. Convert that date/time to the new Epoch time. You will get a big number. For example, for 11-21-2024 0450 GMT (NOV 21, 2024 @ 04:50:00 GMT), the Epoch time is 1732164600.

Again, one Epoch time converter site is: https://www.epochconverter.com

Now, we must convert this number to a 4 byte (8 digit) hexadecimal value.

Go to the hexadecimal converter site and in the decimal field type the Epoch time number. In our example it was "1732164600" for 11-21-2024 0450 GMT.

Using our example, you will get the hexadecimal value: 673EBBF8

The Perseus .WAV file requires this value to be in what programmers call "little-endian format", that is, low value to high value. The value above is in "big-endian format", or high to low value.

We simply need to mirror the hexadecimal number we have. Mirror 673EBBF8, getting F8BB3E67.

We now need to plug in our example number F8BB3E67 into the .WAV file at hex location x34.

Back in your hex editor HxD, set your cursor at hex location x34 (left click just to the left of the location x34 byte itself) and type the new value, F8BB3E67, into the main hex display replacing the 4 bytes starting at hex location x34.

Save the file. HxD will also save a backup of the old file for backup purposes.

Renaming The File

It's not necessary for the Perseus SDR receiver software, but to maintain clarity, rename the .WAV file to the new start time. And since you know the length of the file in minutes, also add the new end time too.

If you have multiple .WAV files, i.e., when a time period is longer than 15 minutes, be sure to change the start time in each file (_001, _002, _003, etc).

That's it. If I get some time at some point, I'll code up a little program to do all this automatically.

Modding Experiments With The YouLoop

The Outstanding Passive Loop Antenna by Airspy

Airspy, the maker of the HF+ Discovery SDR receiver, also makes the YouLoop, the passive, so-called "2-turn" mobius loop sold for use especially with Airspy's SDR receiver lineup.

I bought a YouLoop three or so years ago when I bought my Discovery, tried it for a short time, and put it away, not fully realizing its potential. Sometimes, as in real life, there's more just under the surface than what meets the eye at first. If you don't give a second look, you might miss it. For starters, you have to get your mindset pointed in the right direction, that is, towards ultra-low noise floors and signal-to-noise ratio improvement. Don't stay focused on S-meter readings and brute signal strength. The YouLoop is perfectly matched to present an ultra-low noise floor with maximized signal pickup, and particularly for the mediumwave band.

Factory stock YouLoop

From the factory, the YouLoop has a circumference of 2 meters, comprised of two, one meter sections joined at both the top and bottom, presenting a diameter of 0.64 meters, or 25.2 inches. The top ends are joined through a simple crossover network, interchanging the coax shield with the inner conductor. At the bottom we have a tiny voltage balun to match the loop's low impedance and provide additional noise suppression and isolation. A low noise, highly sensitive receiver is required to get the best out of the YouLoop, to wit, the HF+ Discovery, which has a noise floor on the order of -142 dBm and a minimum discernable signal (MDS) sensitivity of -140 dBm (0.02 uV / 50 ohms at 15 MHz). That is an extremely faint signal. The YouLoop will work minimally with the SDRPlay RSP series of receivers, but they lack the low noise floor and sensitivity to get full value from it. A preamp might help there, mounted right at the loop itself.

In the mediumwave band, the stock YouLoop's sensitivity falls off gradually below 1000 KHz. It was evident that a slightly larger loop might help with this. In attempts to correct this I've done experiments by adding to the length of the YouLoop, first adding two short 1/2 meter lengths of RG-402 (one per side), then two 1 meter lengths in a similar fashion. The 1/2 meter lengths increase the total circumference to 3 meters, the 1 meter lengths increase it to 4 meters. In order to add to the existing YouLoop coax, you will need to purchase two coax pigtails of the same type (RG-402) and two female SMA unions, both readily available from Amazon.

SMA unions and RG-402 pigtails

Let's Do Some Testing

Testing was performed with both extension modifications, comparing each to the stock 2 meter loop while noting the signal strength and the signal-to-noise ratio differences. The standard 6 ft. coax feeder was used between the SDR and the YouLoop and the SDR was connected to the computer through a 15 ft, high quality shielded USB cable to get it as far away from digital hash as possible.

Two distant stations were used as reference and checked during mid-daytime hours, CHLO-530 KHz, Brampton, Ontario (1 KW) at 195 km distance and WWKB-1520 KHz, Buffalo, NY (50 KW) at 111 km distance. Both put in readable but not overly strong signals at these distances.

Results were very interesting, and somewhat surprising, indeed. See the table just below. SNR values are measured to include the carrier + the sidebands.

Freq     Stock 2-meter YouLoop         3-meter YouLoop                4-meter YouLoop   

-----------------------------------------------------------------------------------------------------------------------

 530     -84dB (snr=30, nf=-114)    -76dB (snr=30, nf=-106)     -68dB (snr=31, nf=-99) 

1520    -85dB (snr=37, nf=-122)    -85dB (snr=38, nf=-123)     -83dB (snr=39, nf=-122) 

        *signal strength values in dBFS (dB Full Scale)

        *snr = signal-to-noise ratio, in dB

        *nf = background noise floor, in dBFS

Signal strengths at 530 KHz improved by about 8 dB when the loop circumference was extended from 2 to 3 meters. A 16 dB improvement (530 KHz again) was seen when the loop was extended to 4 meters. To my surprise, signal strengths at the high end of the band (1520 KHz) remained essentially the same for all three loops, rising only about 3 dB (half an S-unit) with the 4 meter loop. The 2 and 3 meter loops produced identical signal strengths at 1520 KHz.

Perhaps more surprising, the SNR, or signal-to-noise ratio of both stations did not vary more than 1-3 dB across all three loop lengths at each frequency. At 530 KHz, all three loops realized an SNR of 30-31 dB. At 1520 KHz, the SNR hovered between 37-39 dB. The relatively unchanging SNR is significant and will allow us to choose the best length loop for our receiving purposes.

The last phase of the testing was null depth. Null depth did not change across all three loops. Differences were undetectable from the original product. I am able to get 23-26 dB null differential across the MW band on semi-local, low angle (60 miles distant) groundwave signals.

So which loop would be the best choice for mediumwave?

The 4 meter loop had only a small gain change over the original 2 meter version at the high end of the band, some 3 dB, but realized about 16 dB gain at the low end. The 4 meter loop seems the logical choice. However, our noise floor at 530 KHz rises from -114 dBm (original YouLoop) to -99 dBm, an increase of 15 dB too! The new -99 dBm noise floor is the equivalent of S-5 on the S-meter. The original YouLoop's noise floor at -114 dBm is about S-2 and much quieter, and with the same SNR and same copyability.

3 meter YouLoop with added pigtails

The 3 meter loop had virtually no gain change over the original 2 meter version at the high end of the band but showed about 8 dB gain at the low end. This time our noise floor at 530 KHz rises from -114 dBm (original YouLoop) to -106 dBm, an increase of 8 dB too! The new -106 dBm noise floor is the equivalent of S-3.5. The original YouLoop, still at -114 dBm (S-2) and still quieter, has the same SNR and same copyability.

The 3 meter loop, the one with the two 1/2 meter extensions, may be the best compromise here. We get signal gain, as it raises signal levels at the low end of the band by about 8 dB. Our noise floor is kept in check at the low end as well, rising only to -106 dB, near a one microvolt signal. The larger 4 meter loop sets the noise floor too high at -99 dB, possibly masking very weak signals at the microvolt level (-107 dBm).

The YouLoop at 3 meters circumference starts to become a little unwieldy indoors, but can be arranged in a vertical oval so that it can be rotated a little easier and not hit other objects in a room. Arrange it in a 24 inch wide by 48 inch tall configuration. Little difference was seen in its nulling ability or signal degradation when configured in this shape.

Conclusions

Extending our original YouLoop to 3 and 4 meters increased our signal levels, but the background noise levels increased by the same amount. Kicking in the preamp on SDR# would accomplish the same thing, amplifying the signal and noise the same amount. The HF+ Discovery has a 15 dB preamp.

The HF+ Discovery's published sensitivity is -110 dBm for 6 dB signal-to-noise ratio. That is a signal some 6 dB, or one "S" unit above the noise. You're going to have to do a bit better than that, however, in that you need a better SNR than 6 dB to extract meaningful audio out of a signal. 9-10 dB might give results, 12-15 dB even better. The signal must be modulated adequately too - low modulation levels will require a higher SNR to make sense of. We can hear a 1 microvolt signal (-107 dBm) if our background noise is enough below that, our bandwidth just right, and our signal is modulated properly.

You need to get the signal strengths up there, closing in on the -107 dBm level to copy audio from signals. In that signal range, it would be best to have a base noise level of not much more than about -115 dBm actual measured noise level.

Note again, we also have the preamp at our disposal. A low noise preamp can make up the difference we need in signal strength. Remember, though, generally the background noise will be amplified equally with the signal. Not to worry if it brings our signal up to a listenable level. Example: Rising out of a -120 dBm noise floor we have a -110 dBm signal we are struggling to get copyable audio out of. Kick in the preamp and we raise that -110 dBm signal to -95 dBm, even though our noise floor is also raised to -105 dBm. The signal is now copyable.

Bonus Test

While I was at it, I put together a tiny 1 meter circumference loop using the two 1/2 meter pigtails. It is the size of a pie plate. One would think this would be so small a loop as to be totally ineffective. Not so.

Here are the test results for that YouLoop configuration as compared to the factory YouLoop:

Freq     Stock 2-meter YouLoop     tiny 1-meter YouLoop !!

------------------------------------------------------------------------------------

 530     -84dB (snr=30, nf=-114)    -97dB (snr=27, nf=-124)

1520    -85dB (snr=37, nf=-122)    -89dB (snr=35, nf=-124)

Signals were lower but adequate, and signal-to-noise levels were in the same ballpark of the other loops. Surprising, even a tiny YouLoop is effective!

Some Low Noise And Signal Improvement Tips

1. Position the YouLoop as far away from your computer and monitor as possible. Monitors in particular are terrible electrical hash generators.

2. Position the SDR itself as far away as possible from your computer for the same reasons.

3. To keep the SDR as far away as possible from your computer, you will need a long USB cable. The USB 2.0 standard allows for cable lengths of up to about 15 ft. You MUST use a good quality, properly shielded USB cable. Stay away from the latest rage, the cotton-covered USB cable. The ones I've tried have inadequate shielding and let all kinds of trash in. I've had great luck with the Monoprice line of cables, available on Amazon. If you attempt to run farther than 15 ft., be aware that you may incur data transmission loss. Your SDR will suddenly drop out of "Play" and into "stop" mode. It may not even start. Trust me, I've tried it. They do make USB amplifiers. I haven't tried them or know if they'd even work.

4. The YouLoop works best when the connecting coax to your SDR is the shortest length possible. Operated passively (without a preamp), you will not get good results or maybe even any results by feeding it with 50 ft. of coax and placing it out in the yard. Try to keep the coax feed under 12 ft. Three to six ft. of coax is optimum. SDRs, like the HF+ Discovery, can also be directly connected to the YouLoop feed point for outstanding results.

5. Used indoors, place the YouLoop next to a window for maximum signal pickup. This really does make a difference.

6. Don't be sloppy with the circle or oval you form. Make a wooden form in the shape of a cross if you like, and fasten the loop to it, ensuring the coax lies in a flat plane. This will maximize the loop's balance and nulling abilities on mediumwave. A child's plastic Hula Hoop also makes a wonderful circular form and can be used in pairs for bigger loop sizes.

RDMW 2024 Mediumwave Pattern Reference

North American Broadcast Mapping Tool & Database

We are proud to announce the launch of the latest edition RDMW-2024 Medium Wave Pattern Reference.

The Radio Data Medium Wave (RDMW) mapping reference is the definitive and most up to date tool for visualizing the broadcasting coverage of radio stations in North America.

Latest features:

• The latest callsign and technical data for all US and Canadian MW stations
• Late summer database cutoff date of August 7, 2024
• Seasonal ionospheric variations included in calculations, targeting October 20, 2024
• Smoothed sunspot predictions included, targeting October 20, 2024
• Current Mexico and selected Bahamas, Bermuda and Caribbean map pins
• Coverage of 490kHz & 518kHz Navtex stations
• Updated skywave formulas producing more accurate nighttime maps
• Real-time night/day terminator (greyline)
• “Click+Save” setting of your receiver location
• Easy tuning control on control bar
• Easy daypart selector on control bar
• Ray tracing control available on a per station basis
• Distance tooltips to all stations
• In-screen “Help” button

Included is a complete set of GoogleMap-based, HTML-driven maps which show the most current pattern plots of all licensed US and Canadian mediumwave broadcast stations from 530 - 1700 KHz. Due to unavailability of Mexican technical data, Mexican stations are represented by pins only this year. Lastly, also included are a sample of Caribbean stations, and the coastal marine NAVTEX 490 Khz and 518 KHz stations. The set includes all frequencies for the indicated services: Unlimited, Daytime, Nighttime, and Critical Hours. Individual maps are grouped by channel frequency x10 kHz: 540, 550, 560, .. 1700 KHz, etc. Data for the plots in this offering is based on the current FCC and Industry Canada databases available at the time of its creation (August 7, 2024). In calculating signal strengths, seasonal ionospheric variations are accounted for as well as smoothed sunspot predictions, targeting October 20, 2024.

Bonus: When you get your copy of RDMW24 you will also receive a copy of William Scott’s interactive greyline mapping tool which overlays on Google Maps. Enjoy!

Click image for the bigger picture.

Daytime pattern example for WAQI-710

Ordering (follow the instructions on this page) https://mwcircle.org/radio-data-mw-rdmw-2024/

Europe's premier medium wave DX club, The Medium Wave Circle will be hosting the download. The Medium Wave Circle is an international club for radio enthusiasts. It brings together people all over the world with a similar interest in medium wave radio (MW, AM or BCB) and related topics.

A small fee will be charged to cover bandwidth costs, less than a price of a coffee. Paid membership in the club is not required. Payment is through a PayPal gateway and is safe and secure. You do not need a PayPal account to use this facility.

Click the link just below and get your copy today.

RDMW 2024 Medium Wave Pattern Reference – Medium Wave Circle (mwcircle.org)

https://mwcircle.org/radio-data-mw-rdmw-2024/

Consider supporting The Medium Wave Circle by joining. Its membership fee is a modest amount for a great return. You will also get access to the group's monthly newsletter, The Medium Wave News, published since 1954, and over 500 past copies of MWN available exclusively to members. This is a 70-90 page newsletter which comes out every month. You will also get access to the Circle archives and pre-publication discounts on the essential reference book the World Radio TV Handbook. Last but not least, you will have access to a great forum of friendly and helpful people on groups.io.

RDMW-24 LAUNCH DATE: 6th SEPTEMBER, 12:00 UTC

User help & FAQ: https://mwcircle.org/RDMW-24-user-guide/

Why MW radio patterns matter to DXers: https://mwcircle.org/north-american-mw-coverage-maps/

We hope you will enjoy this latest version of the Radio Data Medium Wave Pattern Reference. A few hundred hours have been spent this year enhancing the pattern generating program and curating the data. Best of DX!

Checking Signal-to-Noise Ratio The Correct Way

Many of you may be familiar with the HF+ Discovery, Airspy's excellent, highly sensitive, high dynamic range, software-defined radio. I acquired one about three years ago to compare with my SDRPlay RSP1a. There is no competition there, the Discovery wins hands down in virtually all categories.

SDR Sharp, known as SDR#, is Airspy's SDR software made for their product line of receivers, including the HF+ Discovery. It is an outstanding piece of tech and includes many frills like the innovative Co-Channel Canceller which can effectively cancel or null one signal amongst several others on the same channel. I use SDR# exclusively here for reception of mediumwave and shortwave.

Herein we will describe a tip on how to accurately check signal-to-noise ratio using Airspy's SDR# receiving program. More Airspy SDR# receiving tips may appear in this blog, so check back often. I have several of them in mind.

In all the talk about signal strength and signal-to-noise ratio (SNR) lately, the community is finally coming around to realizing that SNR is what makes the difference between copying a station or not. Surprisingly signal strength, usually measured as the strength of the received carrier, may have little to do with it.

Noise floors on today's receivers are high, though there are ways to mitigate this using low-noise antennas and other sundry items in our toolkit. The goal is to set up an antenna and feed system environment with a low noise floor and capture enough signal to rise above that. Don't focus on S-meter readings and brute signal strength. The oft-held directive usually heard is, "Allow the antenna to set the noise floor". Translation: We want enough antenna to capture the signal, but not too much antenna to raise the background noise floor to unacceptable levels.

Signal-to-Noise Measurements

So first, how do we measure SNR? And second, how much signal-to-noise difference do we need? We are not talking about tedious lab measurements here, but simply, "How far above the noise is this signal I'm receiving?". Casual measuring of signal-to-noise ratio while receiving on an SDR is often not done correctly, and sometimes SDR software itself does not report it correctly. It's a value that's interesting to know, and tells us a lot about the signal, our background noise level, and the recoverability of the audio we might be hearing.

To get a sensible reading we must set our bandwidth properly and position it correctly in relation to the signal's carrier. The SNR reported will be the signal power contained within the currently set bandpass compared to the average noise floor also within that bandpass. The measurement problem arises when the station's carrier is contained within the filter bandpass. As we will soon see, the carrier does not convey intelligible information, but its presence may add as much as 20 dB or more to our SNR reading!

So therein lies the mistake: Don't center the bandpass on the carrier! In fact, keep the carrier outside of the bandpass altogether.

dBm Talk, And How We Measure Signal Strength

dBm is the common scale used to quantify received signal power. The dB part, or the decibel, is a logarithmic ratio of one value to another. The 'm' part means relative to a milliwatt, or decibels below (can also be above) one milliwatt expended power into 50 ohms. Let's be clear here about dBm and signal measuring. Most SDR receiving software, and SDR# is one of them, does not have calibrated spectrum scales in dBm. The scale shown is in dBFS, or dB relative to Full Scale. Full Scale is where the SDR's analog-to-digital converter overloads. The meat of our testing here will refer to relative values in dB when comparing loops to each other. That is the important thing to note in the discussion which follows.

Let's talk about the makeup of an AM modulated signal, that which we usually receive on our SDR.

The AM Modulated Carrier

The total AM modulated signal consists of the lower sideband (LSB), the carrier, and the upper sideband (USB). They occupy bandwidth on the RF spectrum. Any time information is impressed upon a carrier it occupies bandwidth. The carrier itself is very narrow, and may only occupy 100 cycles (Hz), maybe even less, depending on how pure it is. All three are easily seen on an SDR spectrum display if we zoom in on the signal a little. Find a signal on your SDR and expand your bandwidth out to about +/-5 KHz either side of the carrier peak and you will see the sidebands fluttering up and down. The audio intelligibility is contained in the sidebands, not the carrier, and can be extracted in total from either sideband. Also note the lower and upper sidebands are mirror images of each other.

AM modulation showing sidebands

If the carrier is fully modulated, called "100% modulation", the carrier (the spike in the middle) will contain 50% of the station's power, and each sideband will contain 25% of the power. If there is no audio signal modulating the carrier, then there will be no power in the sidebands.

To properly measure the signal-to-noise ratio of the signal we're trying to hear, we need to measure how far above the noise the entirety of one of the sidebands is. Thus, we must isolate one of the sidebands in the bandpass.

Here's how, using SDR#:

1. Be sure to use "snap to center" tuning, that is, when you click on the spectrum, the tuning point is always centered in the spectrum window.

2. It's always good to be in DSB (double sideband) tuning mode.

3. Take a look at the zoomed in signal and note how wide the modulation is from the carrier peak to the general edge of the audio variations on either side.

4. With your mouse, grab one of the edges of the bandpass and drag it to reduce the total bandwidth to this edge. It may approach 10 KHz, or +/-5 KHz.

5. Note the bandpass width and roughly divide it in half. Example: If 10 KHz, use the mouse to narrow it further to 5 KHz. We want the width of one of the sidebands. Important! Stay away from encroaching splatter from adjacent channels.

6. Retune the receiver, offsetting the main tuning either positively (for an upper sideband check), or negatively (for a lower sideband check), so that the bandpass contains only the sideband. Important! Make sure the carrier peak is just outside the bandwidth stripe. Try to keep a margin of at least 100 cycles (Hz). It's imperative to not include the carrier in the SNR test. It conveys no intelligible information and greatly inflates the SNR reading to a meaningless value for our purposes.

7. Last, hover your mouse within the bandwidth stripe on the spectrum display. Let the SNR reading settle in for a few seconds. You will see several values. The SNR value will be your signal-to-noise value for the set bandpass width. The Floor value will be the noise floor.

Let's try some examples.

In the example, we see WPIE-1160 in Trumansburg, NY, a 5 KW station at 94 km distance. Our real signal-to-noise ratio is 17.1 dB with a noise floor of -116.6 dBFS. We have 17.1 dB of signal poking its head above the noise. That's plenty, but not overly strong. Our original screenshot of us hovering over WPIE's carrier with the bandpass centered would have us believing our SNR was 44.1 dB. That would be considered an exceptionally strong SNR. But it includes the power in the carrier. Remember, the carrier itself carries no audio information.

Click image for larger version.

WPIE-1160

One more example, WSKO-1260 in Syracuse, NY a 5 KW station at 121 km distance. WSKO is extremely weak, and marginally copyable just out of the noise.  Our real signal-to-noise ratio is only 8.2 dB with an extremely low noise floor of -126.4 dBFS. That is minimal and about the weakest signal which we are able to extract copyable audio. Our original screenshot of us hovering over WSKO's carrier with the bandpass centered would have us believing our SNR was 29.6 dB, under normal circumstances a very comfortable listening level.

Click image for larger version.

WSKO-1260

You will find that signal-to-noise ratios of 9-10 dB to be about minimum for extracting audio intelligibility from a signal. 15 dB becomes comfortable, 25 dB armchair copy.

Remember, centering our carrier in the bandpass is our mistake in measuring SNR. A carrier with no audio modulation or one which is weakly modulated may show an SNR of 40 dB. It's a meaningless value unless you wish to know the SNR of the carrier itself to the noise.

Try this method of checking signal-to-noise ratio. It will give a much more accurate representation of the receiving condition.