ATSC Digital Audio Quality, Technology, Issues

[ota_canuck]

Originally Posted by fordescort

I am experiencing consistent audio "dropouts" on Sun TV (15-1) from my Samsung receiver's digital output to a Techniques audio receiver.

I believe the problem is co-channel interference.

I am in Dunnville and I also have dropouts with Sun 15.1

There is a low power TCT channel 15.1 in Buffalo that often propagates in our direction.

[fordescort]

Is there anyway to remedy this? Also ota canuck, you're about 20 miles from me, are you able to receive ION reliably? If so, could you post your antenna and height. I can get the channel only periodically at night with my CM4228 at 30 feet. I may invest in a tower but can't be sure it will help. Great kids programming.

[stampeder]

fordescort, here's a search tip: check out the Niagara thread and the Hamilton thread and use the Search This Thread tool to the upper right beside Thread Tools and with each try put in a different town name like Dunnville, Smithville, Silverdale, etc. to see how he and others have done and what gear they are using.

[bentoronto]

Live and learn, yesterday I discovered that one of the sub-channels on the local Buffalo PBS station has their FM classical music station (94.5) carried as a "language" and their AM station as another "language." Found this kind of by accident on a website called "remotecontrol."

On my remote control, these are accessed through the button marked "MTS."

First, can somebody help me with the nomenclature.

Second, what are the specs for the audio? It sounds cleaner and crisper than my regular FM radio and I sure can't get Buffalo from Toronto with my current FM radio set-up. Sometimes first impressions of clean-and-crisp are deceptive and based on some funny-stuff audio processing. My Marantz A/V receiver seems to think that audio/radio is broadcast in Dolby (5.1??).

Are there HDTV-only radio stations? Where is digital radio (aside from satellites) - is it hidden in HDTV spectrum like FM is hidden between SDTV channels 6 and 13?

Any helpful discussion greatly appreciated.

[JamesK]

It should sound better because the digital sound system is much better than FM broadcast supports. As an example, compare CDs, which have been around for over 20 years, with what you hear via FM.

The signal is not hidden. It's simply carried as part of the TV signal, which can support mulitple streams. Also, FM radio is just above Ch 6 and well below Ch 7. It's nowhere near Ch 13.

[Jase88]

On the analog side of things, this audio channel is referred to as "SAP" (second audio program). In the digital world, it's often referred to as "alternate audio".

The quality on digital depends on the broadcaster (i.e. simple stereo or 5.1, etc). Though the general quality will obviously be better than analog, as digital transmission elminates much of the noise and interference that one typically experiences with an analog OTA broadcast.

Some digital OTA TV broadcasters carry radio simulcasts as a digital sub-channel. For example, PBS Rochester carries their local University of Rochester station as digital channel 16.7. When tuning this station, you'll get a blank screen and only hear the audio of the radio station.

[bentoronto]

Right.

But what are the specs for alternate audio?

FM is broadcast kind of digital (kind of PCM) and doesn't get demodulated into anything resembling analogue until way downstream in the circuitry. Analogue doesn't automatically mean poorer quality than digital.

BTW, CBC has "English" and... wait for it... "Spanish." Other stations also have weird language naming too.

[JamesK]

^^^^

FM broadcast is most certainly analog. It does not use PCM (pulse code modulation) at all, as it predates common use of PCM. With any analog system, performance is determined by such things as bandwidth, dynamic range, signal to noise ratio, external interference etc. With digital, your quality is determined by bandwidth and the number of bits per sample. As long as you have sufficienct S/N ratio to properly recover the signal, the output will be "perfect".

[gzero]

"Secondary audio" in ATSC would be stored in a separate audio PID (see ISO 13818-1 "MPEG2 Transport Stream"). In a transport stream, each 188-byte packet has a header which (among other things) includes a 13-bit program ID. The PAT (Program Allocation Table) is PID 0x0000, and will tell you the PID of the PMT (Program Map Table). Inside the PMT you will find a table that contains (among other things) groupings of video/audio PIDs. More than one audio PID can be associated with a video PID, and the decoder can select which one to play. As far as audio codecs, MPEG2 TS is pretty flexible -- mp2, mp3, aac, and ac3 are all quite common to find in a transport stream. I would imagine ATSC places stricter restrictions on this, but I'm not familiar enough with it to comment.

[JamesK]

^^^^
Perhaps it might be worth while picking up one of those cheap ATSC converters to connect this audio to your A/V receiver, so that you don't need to have the TV on. You could also use it to feed a VCR.

[bentoronto]

Granted the "definition" of secondary audio is as gzero stated, but hasn't anybody done any empirical evaluations to find what kind of distortion (in the conventional audio family of measurements) results? Seems simple enough to get some sense of the bandwidth, noise, hum, and various kinds of distortions?

FM is most definitely not broadcast in analogue (in the sense of being anything at all like a base-band channel)

The analogue audio signal is coded into something not much different than PCM (OK, maybe we should call it... lets say.... "frequency modulation"... dunno why I didn't think of that before). For the most part, the broadcast signal is treated in a pretty "digital-like" way (amplified into clipping, etc.) and as I said before, the analogue audio signal does not re-appear until way downstream in the circuitry. As such, it shares with digital broadcasting a lot of similar and general resistance (if not immunity) to certain kinds of threats to broadcast quality.

AM, or "envelope modulation" is pretty close to an analogue of the audio signal. The broadcast signal AND the audio signal get battered in correlated and non-recoverable ways while passing through the ether.

Historical footnote: gosh, some of my best and longest-range AM sound was from a 1958 Heathkit crystal radio (that used a germanium diode, of course).

[JamesK]

Digital audio quality is determined by two things, number of samples per second (bandwidth) and bits per sample (dynamic range). As long as the signal is properly recovered, there will be no other limitation, other than quantitizing noise (due to step size). FM radio is definitely analog, as it uses a continuous process (in this case frequency modulation of the carrier) that is proportional to the original signal. PCM, on the other hand is a digital process. The original signal is sampled at a fixed rate (at least twice the highest content frequency) and then converted to a digital value corresponding to the level of that sample. FM was developed in the late 1930's and was used in broadcast TV audio after WW2 (pre WW2 NTSC television used AM sound). PCM didn't see common use until the early 1960's in the telephone network.

[majortom]

Uploaded a .ZIP compressed export from TSReader, for WNED-DT
to Google Docs. I think it should describe the Audio specs of each Program,
which someone had asked for above...Just extract to a temp dir on ur PC, and click on WNED.htm
Lemme know if ya have trouble grabbing it, etc...

http://docs.google.com/leaf?id=0B1k4...g4NTkxOWQ4Njgz

[bentoronto]

Thanks for link but it made no sense to me. Any help?

[majortom]

Starting from the top, Click on a "program" (2,3,or 4 in WNED case)
the hyperlink will take u to the appropiate Video and Audio info in the stream just below the relavent thumbnail

U were specifally asking about the audio specs. Below is what's listed for program 2 of WNED, which happens to be the HD program.
Notice how it tells ya various info regarding how they encoded the AC3 programs. Number of Audio Channels, Center mix level, LFE, etc...

It's not actually gonna tell ya what type of audio program it is (TV, ENG/SP, FM radio, etc), u just have to listen to it to determine that.
But generally speaking they're gonna devote the most bandwidth to the main program's audio (384kbps).

Stream Type: 0x81 AC-3 Audio PID 36 (0x0024)
AC3: Bitrate 384 Kbps Sample Rate 48 KHz
AC3: Mode complete main Coding 3/2 5 L, C, R, SL, SR
AC3: Center Mix Level -3.0 dB Surround Mix Level -3.0 dB
AC3: LFE Mode On Dialogue normalization -24 dB
Descriptor: ISO639 Language Descriptor
Language: eng
Audio type: undefined

Stream Type: 0x81 AC-3 Audio PID 40 (0x0028)
AC3: Bitrate 192 Kbps Sample Rate 48 KHz
AC3: Mode complete main Coding 2/0 L, R
AC3: Dolby Surround Mode not indicated
AC3: LFE Mode Off Dialogue normalization -27 dB
Descriptor: ISO639 Language Descriptor
Language: spa
Audio type: undefined