[ProAudio] Bluetooth Latency (or Baby's First Bluetooth)
engineer_bill at verizon.net
Sat May 15 18:48:01 PDT 2021
A few months ago, I got called by an entrepreneur to fix a design done by a newbie in India. The design put a 50-cent electret mic and a Bluetooth transceiver IC in a battery-powered box to hang in a tree - to transmit bird songs to a receiver inside a home. The client asked me to "fix the background noise." In fact, Bluetooth has a very limited dynamic range (data sheet says 75 dB, with no qualifiers). The mic preamp and 2.4 GHz transmitter are on the same IC - bad idea #1. Then there's the electret mic, whose data sheet makes no mention of noise at all. The original designer made everything worse by putting all this on a 2-layer PCB, making it impossible to contain the radiation from the antenna (1/2" away from the mic input pins) and also did a horrible job of de-coupling the power supply rail. After a re-design to 4-layer PCB, replacement of the electret with an alleged equivalent of an SM57 dynamic mic capsule (from China, of course) at $4, and an added single-transistor low-noise 20 dB gain stage, and improved supply de-coupling, the system - including a cheap Monoprice Bluetooth receiver - is producing an honest 80 dB S/N overall. The project forced me to do a "crash course" on Bluetooth. The audio quality of the signal encoder/decoder or "codec" is roughly the same as MP3 and the latency is high. As Jim says, there's the required (free) version of the codec in every device carrying the brand, and then other "advanced" versions are optional, but IMHO the "improvements" are more about marketing than engineering. If you like MP3, you'll like Bluetooth audio!
Bill WhitlockVentura, CA
From: Jim Brown via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com
Sent: Sat, May 15, 2021 3:45 pm
Subject: Re: [ProAudio] Bluetooth Latency (or Baby's First Bluetooth)
On 5/15/2021 2:23 PM, Mike Rivers via ProAudio wrote:
> If this is typical of BT latency, then so be it. If it should be a lot
> less, like in the 100 ms range or so, then maybe this Monoprice
> transmitter isn't all it should be, or it's broken.
I looked into latency with these systems several years ago. At that
time, there were at least two versions -- a low power, short distance
version with lower latency, and a higher power, longer distance version
with greater latency. Don't take this as gospel, but it's what I
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