"Fixing" audio delay problems in AV systems

First – Define the problem

Do any of these scenarios seem familiar?

• Your analogue RF modulated AV system didn't have any lip syncing issues, but your new digital RF modulation system does.
• Your boardroom has a new projector with HDMI inputs, but when you push analogue audio into the amplified speakers, there's a delay.
• Your new place has HDBaseT distribution everywhere, but when you walk from room to room, the sound isn't in sync.
• Why do different model TVs on a display wall show the same signal at different times?
• I've got digital coax coming out of my TV, running into a DAC and then into my wireless headphones, but the voices are out of sync.
• I can't help you if your lip sync problem is when your backing track fails (cough, Justin Bieber, cough, Selena Gomez, cough)!

Secondly – Understand the cause of the problem

The root cause for each of these is the same – processing speeds. Plug headphones into your phone, press “play” and the sound is right there. On the other hand, pick a music channel on Foxtel, connect the HDMI output to a HD digital RF modulator, push that MPEG4 signal through to a TV, let the TV decode the MPEG4 information, then convert it for the display and the TV's internal speakers. Would it surprise you if there was an audio delay if you stripped the audio from the HDMI output of the Foxtel?

Different TVs, projectors, etc. have different processing delays between video and audio. Different resolutions and refresh rates work better and worse, again, depending on the panels. Analogue audio processing generally takes a lot less time than digital video processing.

Even HDCP problems over HDMI can give you grief when you're trying to sync up audio and video.

Thirdly – Plan to avoid the problem before you plug anything in

Think about each step along the chain from the source to the screen to the speakers. The more things in the middle = more potential problems.

If you have a pub with digital RF modulation, and they want audio through the ceiling speakers, why not put in a set-top box and pull analogue audio from that? It should have a similar processing time to that of the TVs, so its audio out should be synced up pretty well.

Choosing 720p instead of 1080p, 25fps instead of 50fps and so on might be enough to bring all the devices in sync with each other. Your 2007 Pioneer Kuro plasma might have beautiful colours and black levels, but it probably doesn't know much about 4K/UHD.

Think about where you're getting your audio from, in the overall scheme. Stripping audio from HDMI before it goes through HDBaseT connections, or before it goes into a projector is probably going to be a bad idea for syncing. However, using the audio out of your TV/projector would be a good place to try. Keep in mind, of course, that if you use the digital audio output of your screen, there will be another processing stage between the video and the final audio.

Finally – Use tools to match up the audio

Generally speaking, these aren't the best idea. Processing delays can change as the complexity of the video changes, so introducing a static delay might not work well. Still, these might just get you out of trouble.

Behringer FBQ100 Automatic Feedback Destroyer with Integrated Microphone Preamp, Delay Line, Noise Gate and Compressor.

This is a MONO device, so it's best used in a commercial PA environment. Apart from all the other processing effects it has, we're interested here in the delay line of up to 2.5 seconds. Traditionally, you'd use these to allow for delays between loudspeakers in a large venue (i.e. 2.5s ≈ 850m of audio delay).
These come in at a RRP of $239 including GST.

Kramer VA-256xl Audio Delay for Balanced Stereo Audio

This unit is in STEREO, and gives you up to 5.4 seconds of delay in 1ms increments. It uses Euroblock green connectors for inputs/outputs, so you'll have to get your hands dirty with a cable stripper.
A very nice unit if you're desperate, with the RRP at around $1010 including GST.

If you need more than 5.4 seconds of delays, you're not doing it right! Go back to the third stage and try to work out what's causing your delay problems at the source, rather than trying to stick a bandage on the result.