Thursday, October 16, 2008

ATSC vs DVB for North American amateurs

Fred Spinner, W0FMS, posted a comment to an earlier post that I think is deserving of a prominent reply. Here's his comment:

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.

Everything Fred says is true on its face. I still, however, believe that ATSC for amateur TV is not a completely wasted effort.

Although DVB-S receivers can be had for cheap, DVB-T would be the better mode for terrestrial reception (it is, after all, what European broadcasters themselves use). DVB-T receivers not only would be uncommon here in North America, they'd likely not be set up for the U.S. Amateur bands. DVB-S receivers also are going to require some fiddling to get them to work.

But some ATSC receivers are frequency agile enough to be able to work at least on the 73 cm ham band (I'm thinking mostly of computer controlled receivers, like the HD HomeRun or DTV receiver cards). I have some hopes that the HD HomeRun may work on the 900 MHz band as well. We'll have to see. But even if we have to use downconverters to tune alternate bands into the receive range of a traditional ATSC receiver, those downconverters could also be used for analog TV as well. And, as Alton Brown is fond of saying, I look down on unitaskers. :)

We hams have been extraordinarily lucky that for the past 30 years or so cable ready TVs were able to receive 70 cm ATV transmissions completely without modification. I wonder how many TV hams we'd have today if it weren't for the fact that anyone could take a consumer TV, attach a different kind of antenna to the input and tune in the local repeater. With the transition to digital, the chickens are coming home to roost and this happy coincidence is going to go away. Because most (but not all) ATSC receivers are not frequency agile enough to tune amateur bands, we won't be able to tell interested folks that all they have to do is tune their TV to a certain channel. But the opposite side of that coin is that in a few years' time after the transition is complete and the last generation of analog TVs is put out to pasture, there will be a boat load of old ATSC converter boxes that potentially could be modified (perhaps simply by supplying alternative firmware) into amateur TV decoders. There's also all of those PC controlled receivers that are frequency agile.

Fred's last comment about bandwidth is also true - and it is certainly the case that the intent is for the system to multiplex multiple inputs eventually. But you have to walk before you can run. Eventually, I'd like to see the system have 4 video channels: an outdoor camera, an analog TV input, a digital TV input, and a system information slide show.


W0FMS said...

Sorry about the late reply post on this.. I've not checked your blog for quite some time.

I commend your efforts on making ATSC work for ham use. It's a big task. I played with it myself on those Sencore boxes I mentioned previously-- about 15 months ago.

What your are doing is an order of magnitude greater than my "hacks" were.

Still my criticisms stand.

My conclusions on the suitability of ATSC for ham use is based on those experimentations not just theoretically. At a low level and on pre-recorded material I beat you in transmitting a 70 cm ATSC signal by at least a year. But it was across my living room at extremely low level so I'll not count it... ;O) Oh, and I had HD 1080i material running as well... :O)

What I still see as a ATSC disadvantage is the 6 MHz output bandwidth. Unless you mux 4 *useful* streams or some HD or some useful data or other audo streams, then that is wastful of bandwidth. Also PSIP is a waste for ham use, MPEG2 is a waste when MPEG4 (H.264/AVC) is available for "new uses" and ATSC audio is a patent nightmare.

I came across your blog again from a Slashdot post and then the You Tube videos. In one of your You Tube videos you set the encoder to 4 MBPS and say that otherwise it tries to encode at 19 MBPS. I just want you to know that's fine and the video is probably great at 4 MBPS. You do realize that you have about 15 MBPS of null packets (literally zeros) in that stream then? Not good amateur practice in the long run...I have no problems with it again for experimentation, but you are wasting bandwidth configured that way for a repeater output.

What advantage would it be to pick a lower bandwidth mode such as DVB-S or S2 for ham use? Well.. your 4 MBPS video would be on the order of 2.5-3 MHz wide versus 6 MHz. The lower order modulation of 4 states versus 8 would give you 6 dB more S/N. The reduction in bandwidth would also give you 6 dB more S/N (it still works that way with digital signals). Or literally you'd have 12 dB more S/N (greater range or less power needed) and use 1/2 the bandwidth. Go with MPEG4 and reduce the bandwidth used to 1.5-2 MHz and you'd gain another 3 dB. I wouldn't go lower than 1.5 MHz for the trasmission because at that point cheap commercial equipment starts not to work.. but it's certainly possible to do good video in even lower bandwidths.

DVB-S2 8PSK with H.264 at 1 MHz BW would probably do good D1 (480i) video.

I should state your dilemma with where to put the repeater output would be easier with a narrower bandwidth signal as well.

Another issue I see you are hitting is the "patent" issue as well. ATSC audio is Dolby AC3 based. You will never be fully ATSC compliant with Musicam MP2 audio as is standard in DVB-* standards...and apparently what is used on the German boards. You could try to license from Dolby but that is not easy. This is why most of the converter boxes won't work but the excellent HD-Home Run box will (and some other receivers "cheat" on this).

(I own a HD-home run.. awesome tool with TSReader software.. I recommend you get that software as it's not too expensive and does great stream analysis-- there is a lite version available as a trial for free)

Your German modulator boards do the other modes and I *STILL* encourage you to consider other, lower bandwidth modes. The ham bands are much more limited resources than broadcast.

I used to do some analog ATV myself years ago, but I even think 6 MHz analog is wasteful-- but acceptable because there wasn't other substitutes. Now there is, and I still do not think ATSC is the best choice for ham digital TV, cable ready TV's or not.

No matter what I'm still interested how your project goes. I'm curious to see if, with the new chipsets, mobile 8VSB is possible. That was supposed to be the main advantage to COFDM and DVB-T -- that the multipath rejection was superior enough to allow mobile use. (This is why Sinclair broadcasting was pushing it in the US -- they were interested in secondary mobile uses.)

Please continue to keep us informed of your progress.

I wish you continued success on this as I've been interested in doing exactly what you are doing for a few years now.


That Guy said...

I think he meant "formerly used for demos", not "formally"...I was visualizing Best Buy clerks in tuxedos and tails! ;)