Canadian TV, Computing and Home Theatre Forums banner

Why is OTA DTV audio easily broken with weak signal?

5253 Views 17 Replies 12 Participants Last post by  Schmerpy
I use OTA HD, and sometimes I watch a weak channel. When the picture is even slightly garbled, the audio is intermittent. Why?

Audio is 448Kbps Video is 19200Kbps. It is 2.3% of the total amount of data for HD. So why is even a slight garble in video results in bad audio? I understand if bad reception causes me to lose say 10-20% of data, but what's the likelyhood that ALL of 2.3% of audio information is lost in there ALL THE TIME?

For analog OTA, the picture can be ridiculously fuzzy, and the audio would remain crystal clear.

I'm using a LG 50PK550, with the antenna plugged directly into the TV.
1 - 1 of 18 Posts
Actually, almost 43% of the data received by the TV is FEC.

0.8% is sync data (which is used for ghost cancelling)
33.3% is trellis encoding FEC
9.6% is Reed-Solomon FEC

I suspect the reason OTA loses audio before video is precisely because it has less data. So one uncorrected bit error can have a much greater impact.

MPEG-2 video is made up of three types of frames - I frames which are self-contained and P & B frames which are based on past and future frames. However, even within the P & B frames there's a lot of "same as previous frame" (which is compressed down to nearly nothing). So if the decoder was able to construct most of the previous I frame, then it has data it can simply copy to the current frame.

AC-3 audio on the other hand is made up of overlapping blocks of data, so the loss of data in one block propagates into the next block. There's very little ability to try and mask (or simply ignore) errors. (Whereas MPEG-2 video can either copy from the previous frame or produce garbage data.) While a bad block in video will produce an ugly picture, bad audio could damage speakers.
See less See more
1 - 1 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.