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64. Fun With Facts About Latency    by Dave Immer    5/28/15

Sound waves propagate in air at 768 mph. So, if in 1 hour sound travels 768 miles, then in 1 second it goes 0.213 miles (768 miles÷60 min.÷60 sec.) And since that works out to be 1,125 ft. (5280 x 0.213,) the speed of sound, expressed in milliseconds is 1.125 ft./ms. (1,125 ft.÷1000.)

Putting this in a real-world scenario, if you and a colleague were standing on a stage 25 feet apart, it would take roughly 28ms for sound to travel between the two of you. A little more than 1 millisecond per foot. Bear in mind that this is the one-way delay.

If your colleague responds to your sound with his own audio event (for instance two musicians playing together,) add another 28ms for it to get back to you. So there would be an audio round-trip time of about 56ms influencing your collaboration.  For percussionists, this delay would be unworkable, causing each to slow down in response to the “late” arrival of the other’s sound. However, the latency would be imperceptible between two actors playing off each other’s lines, the nature of dialog being much more forgiving of small delays.

Let’s consider the delays we encounter working with popular audio codecs.
Typical encode/decode algorithm latencies:
APTX – 5ms
G.722 – 8ms
OPUS – 5-66ms
AAC – 20-400ms
AC2 – 42ms
MP2 - 107ms
MP3 – 200ms

Remember, with actual ISDN connections you have to add in the physical distance propagation  delay, which in the case of digital signals is roughly the speed of light (186,000 miles/sec.) This works out to 186 miles/ms. So the propagation delay for an ISDN connection of 2 studios on opposite coasts in N. America would be at least 32 milliseconds, round trip. Add to this the codec delays and you’re really having fun with facts about latency.

Comments and questions welcome. 

-Dave