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Thursday, August 2, 2012

The Silicon Labs 477x Chip - A New DSP DX Champ?

Silicon Labs recently released a new "high performance" consumer electronics broadcast radio receiver DSP chip, the Si4770/77 A20 (477x) series. What makes this chip special or any better than the familiar Si4734 chip used in the Tecsun PL-380, PL-310, and other consumer DSP-based receivers? I recently downloaded the spec and programming sheets on the new Si477x series and had a look. This chip set has some interesting new features.

A Sensitivity Boost

One thing was immediately apparent: the new chip has received a sensitivity boost. Where the old Si4734 chip's AM sensitivity was pegged at 25µV (27.9dBµV), the Si477x's comes in at 14µV (22.9dBµV). AM adjacent channel (±9 KHz) rejection for the new chip is 62dB, alternate channel (±18 KHz) is 62dB. AM image rejection is 72dB. Impressive figures for a chip radio.

FM sensitivity has also improved from 1.1µV (Si4734) to 0.66µV (-3.5 dBµV, Si477x). Selectivity is pegged at 65dB at ±100 KHz and 72dB at ±200 KHz frequency offset.

Bandwidth Options Galore

The old Si4734 chip had 7 programmable bandwidth options, the ability to set AM IF bandwidth at various widths from 1 - 6 KHz. Five AM bandwidths were implemented in the Tecsun PL-380: 1, 2, 3, 4, and 6 KHz. The Si477x chip enters with what Silicon Labs is calling "Dynamic AM/FM channel bandwidth control". The AM and FM IF channel bandwidths are dynamically optimized according to the on-channel RSSI (signal strength), and with the ability to further refine bandwidth by detection of the adjacent and alternate channel RSSI. Channel filter bandwidth is programmatically settable from 0 Hz to 15 KHz in 100 Hz steps for both AM and FM. Further, automatic bandwidth selection can be triggered not only by RSSI, but by signal to noise ratio. It can also be hard set to a fixed width by software. This could present some interesting possibilities if full user control is ever implemented in a consumer device.


Uh-oh, Soft Mute Again

Once again, programmable audio soft mute is implemented in the new chip. Soft-mute, a further lowering of the audio level of the received signal when it drops below a prescribed strength, is undoubtedly meant to provide a more comfortable listening experience for the casual listener and not the DXer. The idea is to relieve the listener from all that nasty low level "static" and "interference", or as Silicon Labs states: "....to attenuate the audio outputs and minimize audible noise in compromised signal conditions." Soft mute attenuation in the Si477x is available in 0-31dB steps (default is 12dB of attenuation, unfortunately twice the amount of the Si4734), and can be triggered at a pre-programmed S/N ratio (default is 8dB, the Si4734 was 3dB).

New Audio Bandpass Filters

Silicon Labs has deemed these "Hi-cut" and "Lo-cut" filters. The AM hi-cut control is employed on AM audio outputs having degradation of signal quality. Signal strength and signal-to-noise ratio thresholds can be programmatically set to activate the filters. The Hi-cut filter can be disabled by setting the Hi-cut filter  margin to the default audio bandwidth for AM. AM Lo-cut is employed on audio outputs for rejection of power-supply 50/60 Hz interference. AM Lo-cut is a high pass filter, and is enabled by default and can be disabled by programming. Note that the Lo-cut filter is not available for FM.

HD Radio

For HD Radio (IBOC) buffs, the Si4777 model of this chip series has AM/FM HD Radio capabilities. Additionally, an interesting innovation has been added called "IBOC Blend". This feature supports the ability to blend between analog and digital audio. When the bit error rate of the HD Radio digital signal falls below a predefined threshold and the digital audio fades out, the analog audio is blended in. This prevents the received audio from muting when the digital signal is lost. The audio will "blend to digital" upon reacquisition of the digital signal.

It will be interesting to see if the HD version of the Si477x chip is incorporated in any new, portable consumer electronics device. Car radio seems to be the only outlet anymore for HD Radio, which may die on the vine in the end anyway.


Improved Inductance Handling

Usable antenna inductance has expanded somewhat from 180-450µH to 180-540µH, though the documentation does show the chip capable of maintaining tuning up to a 688µH inductor. Again, like the Si4734, the chip is adaptable to an external air loop antenna if a ferrite rod is not used. An air loop antenna is supported by using a transformer to increase the effective inductance of the air loop. Using a 1:5 turn ratio inductor, the inductance is increased by 25 times and easily supports all typical AM air loop antennas which generally vary between 10 and 20µH.

Frequency Coverage Suffers

Unfortunately, shortwave and longwave band coverage are not available, unlike the Si4734. FM range is 64 MHz - 108 MHz. AM range is 520 KHz - 1710 KHz, at 9 or 10 KHz channel spacing.

Summary

The Si477x is Silicon Labs' cutting edge consumer electronics broadcast AM/FM receiver radio chip. It brings us higher sensitivity for both AM and FM. IF Bandwidth is now dynamically set dependent on signal conditions, using primary, adjacent, and alternate channel signal information. Soft mute is still in force, and is even more heavy handed when left to its default settings. New high and low audio bandpass filters shape the audio signal and are dynamically set by signal conditions. HD Radio for both AM and FM is made available in one version of the chip series. Finally, ferrite loopstick inductance range is expanded.

It is likely that consumer grade AM/FM radios using this chip will stick with default settings for bandwidth and soft mute, basically rendering the increased sensitivity and enhanced DX capability of this radio chip nearly moot. Opening up the myriad bandwidth options to manual selection, allowing soft mute to be manually controlled, and further, allowing the audio bandpass filters to be manually controlled would render this a nice DX machine for both AM and FM.

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9 comments:

HD Radio® Farce said...

Your observations about HD Radio dying on the vine are correct. Retailers have given up on HD Radio as sales were always virtually nonexistent. iBiquity can also forget about forcing HD into cell phones. The only place left is the automakers where HD is mostly an option on expensive Sync/Entune/Navigation systems. BMW and Volvo have it standard but also have outstanding TSB's against HD's many technical flaws. They also share an HD troubleshooting guidelines.

Mark Barnett said...

It's too bad this chip is going in the wrong direction. An AM-FM-SW chip with Digital Radio Mondaile (drm.org) would be forward thinking.

marina jason said...

This is a great article. Thank-you. I often find good articles here.
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Raj said...

Thanks for the nice article. My PL380 goes completely dead with an input signal of 50mV. I wonder if the new chips have similar problems. - Raj, vu2zap

RADIO-TIMETRAVELLER said...

Raj, marina,

Thanks for the warm comments. Glad you liked the article.

Raj,

50mV (if across the antenna terminals) is quite a large signal. The PL-380 should handle 50mV per meter fairly well at sharper bandwidth.

The radio is likely desensing due to strong signal.

It will be interesting to see what the new DSP chips show.

Bill

merlen hogg said...

Hi Radio,
Brilliant article! Thanks for sharing your experiences.

Merlen Hogg
Tormod Helleland

RADIO-TIMETRAVELLER said...

merlen hogg,

Glad you liked the article, and many thanks!

Bill

4nradio said...

Very interesting and well written article!

The mention of automatic variable bandwidth caught my attention. In 2011 I bought a JVC HD car radio (KD-HDR60) that has what JVC terms "Digital Intelligent Advanced Sensing", or DIAS II technology. From the following description it sounds like it also varys the bandwidth according to the signal:

JVC's digital technology helps maximize the performance of regular analog broadcasts. Because unlike analog tuners which are based on hard-wired circuitry, the all-digital DiAS (Digital Intelligent Advanced Sensing) tuner relies on digital programming on a unified chip to deliver versatile next-generation performance. Its advantages include high sensitivity and selectivity with a 16 bandwidth variable Digital IF filter, digital pulse noise canceller for all bands (FM/AM) and intelligent Multipath Noise Canceller for longer stereo reception.

In the radio's menu system you can set the bandwidth to be fixed, or dynamic and adjusted by the radio's DIAS II circuit automatically.

I actually have this HDR60 model in a couple of cars, and I really enjoy its HD radio reception. It's reinvigorated my interest in FM programming and the local Seattle FM scene through the additional music genres and programming compared to the wasteland of standard FM here.

RADIO-TIMETRAVELLER said...

4nradio,

Thank you so much for your comments. It could be that your HDR60 model contains this very chip.

Good luck to you,

Bill
RADIO-TIMETRAVELLER