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Why is OTA DTV audio easily broken with weak signal?

5275 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.
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Analogue TV Standards

There is some discussion in earlier posts about analogue TV. I will explain some aspects of it. Originally from the UK, I was in a TV development lab at the time of the introduction of the PAL colour system.

TV sets in the UK in the mid 1960's were known as dual standard. They had to work on both 405 lines, broadcast on VHF in the UK, and 625 lines broadcast on UHF.

The 405 lines system used in the UK was known as System A. It had positive AM modulation for vision and AM sound. The French 819 lines System E (think of it as early HDTV) was also positive modulation for vision and AM sound and used a channel width of 14 MHz!! The UK 625 lines System I uses negative AM modulation for vision and FM sound.

Positive vision modulation has the sync pulse as minimum transmitter output (almost zero) and peak white at around 90-95% output. Negative vision modulation has the sync pulses at 100% transmitter output and peak white at around 10-20% output - dependent upon the TV system.

In 1936, the 405 lines BBC TV was the first in the world, but positive vision modulation is not the best. To receive the sound properly, the channel had to be fine tuned on the channel selector to put the sound at the correct frequency of the IF amplifier which was 38.15 MHz. Low VHF channels can be subject to interference from car ignition systems which were not as well suppressed as now. This would produce white spots on the video and interference on sound. A white clipper was used to reduce the annoyance of visual interference. On negative vision modulation, any vehicle ignition interference would appear as a black spot and is far less annoying.

The FM sound channel in the TV receiver is produced as a mixing product in the demodulator. This is known as inter-carrier sound. In the UK, FM sound IF is 6 MHz and in North America on the NTSC system, it is 4.5 MHz. Because the output of the vision transmitter never drops below 10%, there is what is known as the residual carrier. This means there is always vision carrier at the demodulator to produce the inter-carrier sound. The FM IF circuit has to have excellent AM rejection otherwise there is a buzz on the sound that varies with picture content, known as inter-carrier buzz. No fine tuning is needed to optimise the sound.

So, to sum up. FM sound can only be used with negative vision modulation as the residual carrier is needed to produce inter-carrier sound. Conversely AM sound can be used with both positive or negative vision modulation.
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