Carrier Sleuthing, Using IBOC Carrier Accuracy

Using IBOC to Calibrate Receiver Frequency Readout

First Some IBOC History

Here in the U.S. and some areas worldwide, iBiquity acquired the rights to digital HD Radio technology many years ago, making them the monopoly in this business. In 2015, DTS, Inc. announced it had entered into a definitive agreement to acquire iBiquity Digital Corporation. Their product is loosely called "IBOC", or "In Band, On Channel". 

IBOC operates on both the AM and FM bands here, and can operate either in a digital-only mode or in a "hybrid" digital+analog mode. Stations can split the digital bandwidth to carry multiple audio program streams (called HD2 or HD3 multicast channels) as well as show on-screen text data such as song title and artist, traffic, and weather information.

Oh Yes, There Were Problems

Only about 240 stations are authorized to transmit IBOC on the MW band in the US. Many less than that actually use it, and this is true especially at night when skywave interference may be prevalent. Since its inception, inherent interference to adjacent channels was found to be occasionally obnoxious. Little wonder, in that the digital carrier sidebands extend fully 14.72 kHz either side of the carrier frequency. These are easily seen on an SDR receiver's waterfall. The nearly 5 kHz portion overlapping into each adjacent channel is only -16 dBc down from the IBOC station's carrier power.

What's left are upwards of 100 stations using IBOC during the daytime, and perhaps 30 at night. Only two or three use the full digital mode at this writing. The rest use the hybrid mode, which impresses the digital signal over the usual analog AM carrier.

But a Bonus

None-the-less, a bonus is to be found in these remaining IBOC stations for us Carrier Sleuthers wishing to calibrate our receivers to within a cycle (Hz) or so. IBOC being inherently accurate in any case, many if not most IBOC stations are also GPS-synchronized, meaning that they are frequency-accurate to 0.02 cycle (Hz) of the nominal carrier channel assignment.

Let's See What AI Says

I asked Claude AI to give us a little more detail on the accuracy of iBiquity's IBOC system. Here is what it had to say.

My Prompt (you're question is called a PROMPT):

Concerning AM broadcast radio stations which transmit using the IBOC standard, is their carrier frequency dead-on their assigned frequency, within one Hz?

Note: Claude searched 4 separate FCC digital standards documents and 7 separate IBOC standards documents. This is the thorough document search I talked about in my post Interesting Questions Posed To AI, information difficult to obtain sometimes and often outside the reach or missed by the average individual.

Claude's Answer:

Based on the IBOC AM transmission specification document, the total carrier frequency absolute error for a GPS-synchronized (Level I) AM IBOC broadcast system must not exceed ±0.02 Hz maximum, and for non-GPS-synchronized (Level II) systems, the error must not exceed ±2.0 Hz maximum.

So to answer your question: No, the carrier frequency is not necessarily dead-on within one Hz. However, it's quite close:

• Level I stations (GPS-synchronized): Within ±0.02 Hz - essentially dead-on
• Level II stations (non-GPS-synchronized): Within ±2.0 Hz - slightly beyond your one Hz tolerance

Most modern IBOC AM stations operate as Level I facilities to take advantage of advanced system features like fast tuning and synchronization with other stations. These GPS-locked stations maintain carrier frequency accuracy that is far better than 1 Hz - they're accurate to within 2 hundredths of a Hz, which is remarkably precise.

Further, major market commercial stations typically implement GPS synchronization (Level I) for their IBOC systems because:

• It provides better performance and reliability
• It enables advanced features like fast tuning
• It's considered the professional standard for commercial broadcasters

The cost of GPS synchronization equipment is minimal compared to the overall transmitter infrastructure.

According to the IBOC specification document I found, the carrier frequency accuracy requirements are identical for both digital and hybrid modes: GPS-synchronized (Level I) broadcast systems must maintain carrier frequency absolute error of ±0.02 Hz maximum, while non-GPS-synchronized (Level II) systems must maintain ±2.0 Hz maximum.

* * * * *

There you have it. I typically tune to WPHT-1210 in Philadelphia and synchronize my SDR or tabletop receiver to within 1 Hz. This is accurate enough to get you in the ballpark for Carrier Sleuthing. Even if you can't synchronize your receiver, you will at least know its offset from what the real frequency is.

So check out HD hybrid digital stations on the MW band, where you can find them. The FCC's LMS database AM Search or even a Google search will reveal if a station is authorized for digital. If you have an SDR receiver, look for an IBOC station nearby to verify if it is indeed digital. Use its carrier to calibrate your receiver's readout. I offer some tips on calibrating receivers in my series Carrier Sleuthing, part#2. See the section "Receiver Calibration".

Good luck!