Monday, September 15, 2008

The difficulties of ATSC

It's a bit surprising where the difficulties of Amateur ATSC TV lie.

It turns out that getting MPEG encoding gear isn't going to be very difficult after all. Part of this is due to the work of European hams, who have been transmitting digital TV using European standards for years now. North American ATSC standards, however, are more challenging.

At first I thought that I'd be able to simply take the output of the MPEG encoder and feed it into the 8VSB modulator, but it appears that the result, though it would be a valid MPEG 2 transport stream, would not be good enough for ATSC. There are three issues remaining that I've identified:

1. The audio has to be Dolby AC3. ATSC doesn't support any other audio format, and AC3 is heavily patent encumbered, meaning there are no low cost encoders for it. You can encode it with open source software, but now you're using a PC as your encoder, with all of the complications and reduced reliability that entails.

2. ATSC requires a constant 19.39 MB/s data rate. You're supposed to stuff the channel with null packets to make it so if your data is less than that.

3. The dreaded PSIP. PSIP is a complete waste of time and effort for amateur TV, but if you want the thing to decode properly with consumer grade tuners or receivers, you've got to generate at least a static set of PSIP tables.

On top of all of that, one other ham I've talked to who is extremely experienced with ATV says that 900 MHz and 1.2 GHz in the bay area are more or less useless due to heavy interference. And if that weren't enough, there's 8VSB's sensitivity to non-linearity in the RF chain, which has implications for the design of your power amplifier.

European hams have had success with DVB-T, which doesn't have these issues, but you can't use ordinary North American consumer equipment to decode it, which makes it (at least here) just another specialized ham-only mode. The value of attempting to transmit ATSC compatible video is that just adding a downconverter to a standard consumer system is the only change required, which expands the potential receiver pool exponentially.

This is looking less possible daily. :(


G7LTT said...

DVB-S is what they are using in the UK/Europe. This is available over here too from the many FTA satellite RX providers such as Pansat.

I realise that the US has a tradition of using domestic TV sets for ATV reception but with the costs involved and the hassles associated with ATSC why bother going to all that expense?

Many projects are available to transmit MPEG2 format TV pictures via homebrew equipment (which is where most ATV is done) if one were to look around.

Mark, G7LTT/NI2O

Nick said...

There is some value to being able to use consumer receivers with nothing more than a downconverter for ATV. The big one is that it dramatically lowers the participation barrier. It's hard enough to suggest to folks that they buy a downconverter so that they can tune into a mostly empty repeater, but if you then tell them they need to buy hundreds of dollars worth of gear to even just *receive*, you're not going to get many participants.

Fred Spinner, W0FMS said...

A okay quality DVB-S receiver can be had for far less cost than a PC electronics downconverter. All that likely would be necessary is a preamp at the antenna. I've seen PC card DVB receivers go for $10 and used DVB-S receivers go for $30. New ones, non high def, are less than $100 at the low end.

The disadvantage of ATSC is the complexity and the fact that you are transmitting on a 6 MHz channel. A DVB-S transmission can be done in good quality at about 2 MHZ. Also QPSK isn't going to be nearly as touchy as 8VSB as far as linearity is concerned. I've experimented with both-- I have an e-bay ATSC modulator/transmitter (formally used for demos at "Best Buy") and I find the fiddling that has to be done with ATSC (such as PSIP) data to be a big waste of effort.

The only disadvantage with DVB-S is multipath as QPSK/Satellite isn't really designed for that.. but if you are going to use gain antennas on both ends who cares? The MPEG-2 DVB-S or S2 standards are also far more lenient of audio and video rates. I suppose if you'd like to be cutting edge DVB-S2 and MPEG-4 H.264 would even be better than ATSC.... It's possible to do HD at low rates with H.264 and still only use 2-3 MHz of bandwidth.

I do hope if you are going to use ATSC for a repeater output that you strongly consider multiplexing multiple input receivers.. otherwise you are literally wasting the spectrum that the null packets consume.