Working with Latency in Digital RF Applications

by Doug Engstrom
November 1st, 2016 in Product Tips

Latency is the delay between the video source and the time it takes to appear on the TV or screen. For live presentations from an on-site video switcher, the delay is usually due to scaling and rescaling the video, as well as processing delays in switching and effects. This can typically range from nearly instant to a few seconds.

In digital RF and IPTV, an HD program is encoded into an MPEG stream on the fly, sent through the RF coax, then decoded in the TV or HD tuner. We call this the end-to-end latency - the total time from source to the screen. The latency in this process will be longer than baseband video switching - as the encode/decode process takes time. For some digital RF modulators, this process can take over a second - some reduce that delay by sacrificing video quality.

The delay is composed of three elements, time to encode the stream, time in internal buffering, and time for TVs to decode the stream. One benefit of digital RF is that delivery over coax is instantaneous, where sending the same MPEG stream through Ethernet can take up to 8 seconds. Ethernet packets only guarantee delivery, not how long it takes.

If you have existing QMOD Encoder-Modulators, you can add low latency by downloading the free QMOD Update from our website. The intelligent Windows app works for all models, so only one download is needed.

EXAMPLE: If you are feeding 1080i SDI to our older QMOD-SDI HDTV Modulator, you can cut the delay in half to about 330 ms converting 1080i to 720p using a Decimator HD-MX or other SDI scaler.

That said, be aware of the inherent latency for MPEG streaming. When the speaker is presenting right in front of the screen, latency is critical - it's unnerving to see the speaker's hand move down, then see the hand move down 50 ms later on the screen. Even more so a half-second later. Near-instant latency is hard enough to achieve with baseband video systems - it's not possible when you add MPEG encode/decode delay.

There are other live applications where a half-second delay isn't as much an issue. The delay works well for large arenas and stadiums where the typical delay for digital RF is over a second, and IPTV averages about 8 seconds.

There are solutions for some applications. The first option is to use RF distribution for the application it’s designed for – remote distribution where latency is irrelevant.

There is a solution for the typical sports bar/ hotel installs – using an audio delay of 500 ms between the source and the audio amplifier. Typically, this will be used for one channel that feeds a video projector or video wall.

Typical products include: