Before the Federal Communications Commission Washington, D.C

Before the Federal Communications Commission Washington, D.C. 20554

In the Matter of: ) ) All-Digital AM Broadcasting ) MB Docket No. 19-311 ) Revitalization of the AM Radio Service ) MB Docket No. 13-249

Comments of David L. Hershberger March 9, 2020

Introduction

The Commission proposes to allow all-digital AM broadcasting. While this in itself is not a bad idea, allowing the Xperi “HD Radio” MA3 mode system is. It occupies three channels instead of one, it is unsuited to the propagation of medium wave signals, and it has an inefficient, obsolete audio codec which cannot be upgraded. DRM occupies but one 10 kHz channel and is better suited to medium wave propagation.

Bandwidth

Any all-digital transmission system must be compatible with the allocation structure. And that means occupying just one 10 kHz channel instead of distributing energy across three. MA3 (all digital) is just as bad as MA1 (hybrid) when it comes to adjacent channel interference. The secondary and tertiary sidebands of MA3 are transmitted at the same level as the primary sidebands of MA1. The only difference is that the MA3 mode trashes the opposite sideband of the MA1 mode.

MA1 already causes interference for me. We live at about 3700 foot elevation in the Sierras. I find that the weather reports from KKOH Reno are more pertinent for our elevation than what I can hear on the Sacramento radio stations. KKOH is 50 kW on 780 kHz and 66 miles from my house. Unfortunately, KCBC is 50 kW on 770 kHz in Manteca, CA, some 103 miles from my house. KCBC operates MA1. It adds an annoying hiss to 780 kHz. When driving through the Sacramento and Roseville areas (99 miles from KKOH, 70 miles from KCBC), the MA1 noise from KCBC makes KKOH unlistenable.

In paragraph 17, the NPRM states, “By design, all-digital AM is less likely to cause interference to adjacent channel signals than hybrid operation, due to the relocation of the digital carriers to the center of the channel.” I disagree with this statement. In my example, MA1 on 770 kHz trashes 780 to 785 kHz. MA3 will trash 775 to 780 kHz. This makes no difference. Interference to the lower sideband is just as harmful as interference to the upper sideband. MA3 will provide no relief to those suffering from MA1 interference.

1 There is no need for digital systems to operate as spectrum hogs. Any all-digital transmission system must be limited to 10 kHz bandwidth. There is such a mode available from Xperi but it is not being proposed.

DRM offers a 10 kHz bandwidth mode, and should be allowed.

Medium wave propagation

The AM broadcast band enjoys skywave propagation at night. Skywave allows some stations to have very large nighttime coverage areas. Any allocation or deployment of technology must consider the basic physics of propagation.

The proposed Xperi systems generally do not work with skywave propagation, or more accurately stated, selective fading. This makes them misfits for standard (AM) broadcast. Any digital AM system must work with skywave propagation and selective fading to be a good match between modulation mode and propagation modes. This would be a huge waste of spectrum, because beyond the groundwave propagation area, the Xperi signals would become nothing but interference. At night, distant signals would generally not be receivable, and would just be noise.

DRM, on the other hand, was originally designed for use on HF (shortwave) frequencies which are entirely skywave propagation with selective fading. DRM is a much better match for medium wave digital.

Upgradeable codec

In my opinion the Xperi codec used for AM is horrible. Sure, it’s nice to hear wide bandwidth on an AM signal. But after 10 minutes, fatigue sets in. The codec sounds robotic and mechanical. The artifacts are highly objectionable. I would rather listen to 3 kHz analog.

Having a strong digital signal makes no difference. You get the codec artifacts with strong signals just as well.

There are much better audio codecs available today. Opus is an open format codec which works well at low bit rates. https://en.wikipedia.org/wiki/Opus_%28audio_format%29

Codec technology still seems to be advancing faster than modulation technology. Any all-digital system must also be upgradeable. The audio coding algorithm in particular should be upgradeable. As new technologies enable more efficient use of a 10 kHz channel, they should be deployed to existing receivers, hopefully without any intervention required by the listener. AM digital receivers should be made to accept over-

2 the-air upgrades. In other words, any rulemaking for digital AM should specify the receiver, and not any particular audio codec.

Carrier frequency tolerance standard

Included in the NPRM is the notion of tightening the archaic 20 Hz frequency accuracy to 1 Hz. This is good, but a 1 Hz frequency error is near the point of maximum annoyance for analog listeners. A 1 Hz error is slow enough for receiver AGC to follow, pumping audio levels up and down. Higher frequency errors (such as 20 Hz) produce beat notes high enough that receiver AGC pumping is lower in amplitude.

One millihertz is easily achievable with surface mount GPS modules, which cost as little as $30. This would reduce the error below the point of maximum annoyance, which is around 1 Hz. I suggest a 0.001 Hz limit. This should really clean up the AM “graveyard” frequencies at night, as well as all the other channels.

Finding a new home for “AM”

Until and unless electrical noise in homes can be radically reduced, AM radio will be heard mainly in cars. So what can be done to make “AM” stations audible in homes?

One solution would be to simulcast AM programming on audio-only channels in ATSC 3.0 TV broadcasts (and/or ATSC 1.0 using the M/H mobile/handheld standard). If ATSC demodulators are included in radios, the selection of either analog AM or the corresponding ATSC 3.0 audio channel could be done automatically and possibly invisibly to the user. This could be done in both home and vehicular radios.

Conclusion

Xperi needs to be sent back to the drawing board. Their system sounds bad, locks in obsolete technology, does not work with medium wave skywave propagation modes, and it occupies far too much bandwidth. It does not meet the basic needs of a medium wave broadcast system.

On the other hand, DRM in its 10 kHz bandwidth mode makes a lot of sense. It works with skywave. It includes ancillary data (such as images).

Either system could be improved by making audio codecs upgradeable.