[mythtv] pcHDTV oddity

Jarod Wilson jcw at wilsonet.com
Tue Sep 14 13:45:25 EDT 2004

Doug Larrick wrote:

> Jarod Wilson wrote:
>> Hey all,
>> 1) CBS in my area has one of the strongest signals, and only one 
>> sub-channel. If I tune to it in live TV, 9 times out of 10, its 
>> perfect. The other time, the picture is fine, but the audio sounds 
>> like its muted and underwater. I can live with that, but *every* 
>> recording (out of like 7 so far) done on that channel has the 
>> underwater audio problem.
> Could it be that CBS is sending two audio programs, and one of them is 
> broken?

Doesn't look like they're sending two audio programs, but you'd know how 
to interpret this better than I. (see below)

> Do you have access to another HDTV tuner that can switch 
> between audio tracks?

Nope. :-(

> You could run my 'testinfo' stream-info program 
> (<http://jekyl.no-ip.org/doug/testinfo.C>; 'testinfo /dev/dtv | more' 
> after doing dtvsignal to change channels).  Look for the TVCT 
> (Terrestrial Virtual Channel Table) and see if it has multiple audio 
> streams listed.

Okay, here's the TVCT stuff from testinfo:

************PSIP table type TVCT (Terrestrial Virtual Channel Table) size 62
Subchannels: 1
Program #1: KIRO-DT, channel 7-1, source_id 1 **
Base PID 0011:
   PID 0011, type video, language:
   PID 0014, type AC3 audio, language: eng
Unsupported proto version 255

And for good measure, here's the output from dtvstream:

$ ./dtvstream -c 39 -q -v
Tuned "/dev/dtv" to channel 39.
Lost sync at input offset 0
Achieved sync after 79 bytes.
Summary after scanning first 20000 packets:
     3 MBytes processed @ 15 Mb/s
     54 pes_pkts processed
     60 atsc_pkts, w/ distribution:
         sst=2/2, mgt=52/1, vct=6/1, rrt=0/0,dcct=0/0, dccsct=0/0
     Dropped pkts (Continuity Counter Errors) = 0
     Active PIDs in this stream
         PID=0x0, ctrl_type=PAT, pkt_cnt=15
         PID=0x10, ctrl_type=PMT, pkt_cnt=16
         PID=0x11, ctrl_type=VIDEO, pkt_cnt=15074
         PID=0x14, ctrl_type=AUDIO_AC3, pkt_cnt=437
         PID=0x1d00, ctrl_type=EIT, pkt_cnt=3
         PID=0x1e01, ctrl_type=ETT, pkt_cnt=1
         PID=0x1ffb, ctrl_type=ATSC, pkt_cnt=60
         PID=0x1fff, ctrl_type=NULL, pkt_cnt=4394
Stream contains 2 total program(s).
Stream contains 1 valid program(s).
     Using program 1: 7-1 KIRO-DT:
         Video (MPEG-2)
         Audio (Dolby AC-3), language = eng

Maybe something to do with the 2 total programs, but only 1 valid? I 
haven't a clue what the "Unsupported proto version 255" message from 
testinfo means either. Slightly wonky audio stream perhaps? Odd that it 
works 9/10 times in live TV but always fails on recordings though...

> There was some work done recently to allow Myth's libavcodec to deal 
> with MPEG streams containing multiple audio programs, and switch among 
> them.  It might be worthwhile to pass (record) *all* the audio streams 
> for a program in hdtvrecorder and see how well this works.

I'm assuming from the above output I'm only getting one audio program.

>> 2) ABC also has a very strong signal, and worked great several months 
>> back when I last had my card working. I'm not sure if its related, but 
>> this is my only 720p station... It always plays back like its in slow 
>> motion, with stuttering video and audio, both in live TV and 
>> recordings. If I pause the video, then unpause, it seems to play fine 
>> for a sec, then starts crapping out again.
> What is your output device?

GeForce FX 5200 VGA output to Audio Authority 9A60 transcoder, custom 
mode inside 540p timings fed to a high-def TV via component video. I 
still need to get around to creating a viable 1080i mode...

Mode "in540p"
         DotClock 37.26
         HTimings 880 944 1048 1104
         VTimings 480 506 520 563
         Flags "+HSync" "+VSync"

> What refresh rate?

VertRefresh locked on 60.0 (hm, haven't tried 59.94...), HorizSync at 33.75.

> Can you post output of 
> 'mythfrontend --verbose playback'?

Not at the moment, but I certainly will once I have an opportunity 
(probably end-of-day). Thanks Doug!

Jarod C. Wilson, RHCE
jcw at wilsonet.com

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