Audio Dropouts - ADB 3800C HDMI

Since the very beginning of the Starhub HD trial, I faced an audio dropout issue. Every few minutes, the audio would drop for 2 seconds or so, then resume. After the single firmware upgrade available, I still face the audio dropout problem, but less. This problem occurs in HD, and SD channels too. Just to clarify, this issue ONLY happens for me if I use HDMI audio. I did not try the AC3 out - but the stereo jacks don't give the issue.

The last time I faced such an issue with digital video, in digital video editing, this is the problem - I'm not saying that the SCV HDMI audio dropout problem is exactly this problem, but perhaps the problem I faced years ago can raise some ideas on how to troubleshoot among the SCV people.

The old steadfast Sony M1 DV codec chip used in some video editing systems, supports a standard called 'locked audio' in DVCAM systems. In locked audio format, the audio output must be set to exactly 32032hz per second, otherwise the chip will not accept the audio in sync. The Sony consumer DV cameras are also designed to record/output audio in and around that frequency, averaging around 32032hz for every second.

However, in the Canon XL1 the audio frequency changed heavily for every second, sometimes with differences of more than 20 to 30hz between seconds. This caused the average audio frequency was around 32053hz for every seconds. When Canon 32053hz audio mixed with Sony 32032hz audio, in a video editing system using the Sony M1 DV codec chip, the audio sync will be way off, and unusable. The symptom is EXACTLY what I see in the Starhub HDMI audio - dropouts at irregular intervals. I remember the dropouts very well, the resemblance is uncanny. The dropouts will happen once the sync goes to hell and the audio buffers reach their limit, the chip will try to flush the buffers to reset the sync, causing an audio dropout in the process.

The solution: When capturing, the video editing system will look at the first second (store it in the buffer) and measure the sampling rate, and if that sampling rate was outside the range of 32026hz to 32038hz, the editing system will consider the camera as an XL1 and capture audio as 32053hz per second. After sampling as 32053hz per second, it will convert the audio to 48048hz, and then convert back to 32032hz, to create an .AVI file. This is to create an .AVI file that is compatible with other cameras.

Credit to Charles Burmeister in DVinfo.net in jogging my memory.

What this means to Starhub broadcasts, is that the sampling rate of the audio streams broadcasted by Starhub's DVB-C system, may be `slightly' out of the max and min parameters which the set top box's HDMI transmitter can handle. OR, the ADB 3800C box does internal audio conversion (HDMI stage), and outputs the audio via the HDMI which sampling rate are out of the min or max parameters of the TV's audio system can take, again causing a sync issue. Or both, of course. Both ways, it points to the HDMI transmitter chip - since stereo jacks don't have an issue. I've yet to experiment with the AC3 optical out, but Starhub guys can do that, I don't have time for that now.

Because the sampling rate may not be fixed, this will create dropouts not at regular intervals, but irregular intervals. It can range from hours without a dropout issue, or just 30 to 40 seconds.

Starhub officials who see this, may it give you a little idea on how to fix it. Worst is that the ADB box has a stability issue with the system clocks.

If it is indeed an audio sampling issue, the following problems can be eliminated:

1) TV's AC3 decoder problems - cannot be, because everybody here have different TV, and most experience the issue. Anyway, our TVs work fine with the AC3 output from our HDMI DVD players.

2) Source video issues - cannot be. All your source is like that. Worse, I am experiencing lipsync issues with your mediacorp content! Which I did not experience with the old Digital set top box or analog set top box.

3) Cable issues - cannot be. I used 3-4 different HDMI cables already.

4) Signal Strength - Nah. I have 100% quality with very low Biterror rate according to the Starhub contractor.

It might be a HDMI transmitter issue (most probably Silicon Image)... other devices have this issue also: see http://forum.ecoustics.com/bbs/messages/2/162876.html

Anyway, over to you, Starhub.

Edit: With more research, so many similar problems on the net, just search for `HDMI audio dropout' in google. I am indeed quite convinced that the HDMI transmitter chip on the ADB 3800C has some issues. Refer to this link here for best evidence. I quote:

Observation; audio would drop every few seconds and "digital" would blink on amp indicating no optical sync at all. Each time audio would come back there would be a slight glitch in a part of the picture as well. Appears to have been recorded this way as I could replay the problem.

So every audio drop, optic sync is `gone'. The HDMI transmitter is probably emptying its buffers. Then the sync comes back. C'mon, this is a good lead for the troubleshooting!

And I urge Starhub to try with AC3 out (optic) and see whether you have the same issue or not. I would expect that the AC3 out has no issues. If so, wham, you got your culprit - the HDMI transmitter.