I posted a blog entry about a month ago about an issue that started appearing after customers install Update Rollup 2 for Media Center 2005. Sometimes, while playing back live or recorded content, Media Center will stop displaying video and show a blue overlay screen with the text Digital Audio Service. When I originally posted my previous blog entry, I had only heard from 2 people who were running into this issue, and both of them were able to resolve the issue by updating video and TV tuner drivers on their systems. Since then, I have heard from many more people, and updating drivers has not helped in most cases (as you can see in the comments on that previous blog post).
Unfortunately, I don't have an exact root cause or fix to pass on yet, but I have been able to learn more about what we think is happening in these scenarios. Some of this is still speculation because we are trying to reliably reproduce this issue in our test lab and are so far basing our investigation on the diagnostics reports that some of you have been kind enough to send me so far.
There is an algorithm in the stream buffer engine (SBE) that detects dropped frames and attempts to synthesize replacement frames to minimize glitches and freezes while watching video from analog signals. Once the dropped frame rate gets high enough, this algorithm stops being effective because there are not enough good frames to use to synthesize replacements from and we will halt video playback. Media Center sets a threshold for dropped frames to decide when to stop trying to create replacement frames in this scenario.
The Digital Audio Service overlay message is a new message introduced in Update Rollup 2. It is designed to catch and report low frame rates in DVB-T signals as opposed to letting the video glitch and freeze. There is a different threshold that controls when Media Center decides to show this Digital Audio Service overlay than the threshold I previously described in the stream buffer engine for analog signals. It appears that the threshold for this frame dropping rate is set lower than the frame rate threshold for the stream buffer engine for analog signals and therefore Media Center shows this error prematurely in some cases.
Possible causes for high dropped frame rates include the following:
- Outdated drivers
- Older tuner cards
- Non Media Center processes running on the system that are CPU-intensive (because processor time otherwise used to process video frames is being spent elsewhere)
- Weak signal strength
In cases where the Digital Audio Service overlay appears, you will see that playing back the same video in Windows Media Player will work because WMP does not have this frame replacement algorithm or varying thresholds. However, it is likely that you will see some type of glitching around the same time in the video playback as you start seeing the overlay in Media Center.
The frame dropping thresholds that I mention above are all controlled within Media Center code, so unfortunately that means there is not a way to configure a registry setting to make the Digital Audio Service overlay appear less frequently or never.
We will be spending more time trying to isolate a repro case and determine possible fixes or workarounds for this issue and I will post additional updates as I get them.