The Month in BEM, January 2010: Correcting Lip Sync Errors
I recently received the January issue of Broadcast Engineering Magazine. I love this magazine. These guys are looking at the stuff I am. I am constantly learning something new! Everyone should get the magazine or follow it online.
So, I opened the magazine anxiously, to see what gems of insight awaited me. One article quickly caught my eye: Correcting lip sync errors
MMmmm! I must admit, my mouth started watering. (Sad, I know, but truthful, nonetheless.) I devoured the article, and then…I devoured it again. Usually after all that devouring, I’m ready to sit back and digest. The problem was that I was still hungry! There was no meat I could sink my teeth into. Where WAS the beef!?!?
The thought of everyone reading the article and feeling the same way as I did, provoked me to post a comment to the article online. I have no way of knowing whether they will post it or moderate it, but since it was requested here also, I am posting my comments and solutions.
To see Mr. Cugnini’s article:
Correcting lip sync errors | Potential remedies to this frustrating problem are within our reach
My comment I’ve called “A View from the Receiver Side”, follows:
Dear Mr. Cugnini,
Thank you for your articles and insight concerning the digital transition. I have enjoyed them very much!
Your article “Correcting lip sync errors: Potential remedies to this frustrating problem are within our reach” though, left me reaching and frustrated. As I read your article, I began to think that you and I are viewing the lip sync problem from very different angles. I would like to offer…
“A View from the Receiver Side.”
When diagnosing lip sync problems in the home, just like medicine, only the correct diagnosis will end with a cure.
In your article, you describe 3 causes of lip sync.
1. Broadcast error
2. Decoder incompatibility
3. All other
In your comment “In order to solve the A/V sync problem over the complete extent of the content distribution chain, attention must be paid to equipment, including consumer electronics.”, you pinpointed the area of this discussion.
When diagnosing lip sync issues in the home, the list of diagnoses look like this:
1. Broadcaster error 1%
2. Decoder incompatibility 1%
3. Equipment failure (heat, etc.) 8%
4. Errors in installation 90%
I don’t know what issues you face in the broadcasting arena, but the view from the receiver side is that broadcaster errors are only a very small amount of the problem. As far as broadcasters are concerned, I believe that they are doing a great job! Decoder error must also be a similar small percentage of the whole, again accounting for just 1% of the lip sync errors, and when added to broadcaster error, make up only 2% of lip sync errors. These are incurable at the home system.
The remaining 98% of lip sync issues in the home are curable! Let’s look more closely at the remaining 98% of errors.
Defective or failing equipment accounts for the 8% of the 98%. (Faulty receivers are not included in the coder incompatibility category. Coder incompatibility is software fix and out of reach, as you suggest.) These are receivers/equipment that are defective, and due to heat failure or other reasons produce errors in lip sync. For this 8% the cure is to replace the faulty piece of equipment.
So what of the other 90% of homes with lip sync errors?
These errors are all generated from poorly or incorrectly installed systems.
The source of timing errors in these systems is corrupted or lost data within a signal stream. Specifically, it is the corruption or loss of missing timer information. The PCR, PTS, and DTS are normally available in the stream as you said, but in circumstances that cause degradation of data, it is sometimes the timer data that gets corrupted.
Data stream corruption due to improper installation at the home happens far more often than can be attributed to corruptions from the source or atmospheric conditions.
When dealing with lip sync, first we must determine the cause. In most cases, individuals (not entire markets) report continued error in lip sync. If the receiver is functioning correctly (as good as it can with what data it receives) we rule out broadcast error because this is infrequent and everyone would have the same complaint, and we rule out the decoder incompatibility because this would be stable and continuous for a particular station or program. Any other corruption of signal may cause a lip sync issue, depending on what data is corrupted. Occasional lip sync problems, in the infrequent case, are a symptom of a separate problem that occasionally exhibits itself as the lip sync symptom. Lip sync can be a symptom of the general problem of corrupted signal.
When lip sync is a regular occurrence at home that is not limited to a specific broadcast or channel, or when the primary complaint is frequent lip sync error, then we have a solution.
There are two primary means in which data gets corrupted in the home system. One is low SNR (signal quality) arising from poor alignment of the receiving antenna. While this causes ANY and all types of error in a system, the errors will not specifically (or consistently) manifest as lip sync error. In some cases of lip sync, a proper alignment, where signal is maximized, will cure the problem.
If the lip sync problem remains, you must then address where lip sync error most commonly occurs in home systems. Lip sync most commonly occurs in coaxial transmission lines when the center needle is too short in a connector or fitting. Loose fittings can also contribute to lip sync problems.
You can cure this by examining the cable connections to ensure that the center conductor protrudes a minimum of 1/16th inch from the fitting, and tightening all connections. Following this procedure (maximize signal and maximize connections) will result in the solution to lip sync problems for the largest number of cases.
Lastly, you ended with this comment: “Of particular interest is the discussion of packet timing, which can aid in the understanding of similar mechanisms in professional encoders and multiplexers– but that’s another topic.”
Maybe that is another topic, and perhaps the errors at the station are related to packet timing, but the cause in the home, is corrupted timer data.
Lip sync occurs when the center conductor is too short. This short needle creates an air gap that needs to be crossed. Lacking conductive surfaces, the minute electrical charges (normally representing digitally modulated information) need to build up to use their electrical potential to spark across the gap. This release of electrical energy either corrupts the digital information (flips or changes some of the bits), or more likely, destroys the info (packet). To obtain the spark needed to cross the gap, the initial or first data contained in the stream, is converted or destroyed as it crosses the gap. Timer data is some of that initial or first data that is received.
I hope this helps everyone in their solutions and studies of lip sync errors.
Jeffrey C. Johnston
Tell me what you think. If someone has additional information, please share!