embedded audio mixing project - Page 1
Forum section: electronics / general

author: Harvs
author_rank: Super Contributor
timestamp: July 19, 2014, 05:26:06 am
content: I'm looking at building an audio system for long distance riding (motorbike.) Having bought and used a few different systems over the years, some of which cost quite a bit of money, I've come to the conclusion that the folk that design and build these systems really don't do the sort of riding I do, they've all had some fatal flaws.At the heart of the application, what's needed is to take a bunch of audio inputs, mix them in various combinations and levels depending on what's happening, and output them to various places. Add to that perhaps some basic filtering to improve the quality of mic inputs, not really sure until I try that one out.There's the whole user interface and communication side to it which I think is a no-brainer to use some flavor of cheap ARM linux board to make it easy. But for the audio processing I'm thinking of having a second board to do it all with a bunch of ADC/DACs and programmable frontends to set analog input levels.There's quite a few channels, albeit it doesn't need to be anywhere near hifi:Inputs - rider mic, pillion mic, UHF, MP3 (stereo), Radio, GPS Nav, Phone, etcOutputs - rider (stereo), pillion (stereo), UHF, Phone, etcI'm thinking of perhaps an XMOS processor for it, as they have a dual stereo ADC/DAC card for the starter kit that would be a good starting point for testing ideas. Or an FPGA could probably do it pretty easily as well.I haven't actually used an XMOS before (or done anything with I2S), any foreseeable reason why one wouldn't be able to do this? Or is there a different processor that would make more sense?

author: nctnico
author_rank: Super Contributor
timestamp: July 19, 2014, 08:08:03 am
content: I'd look for codecs for use in telephony systems (8ks/s 16bit) with an SSI interface. The advantage of an SSI interface is that you can daisy chain several codecs and there are several ARM controllers (NXP for example) which can interface with SSI.

author: Precipice
author_rank: Frequent Contributor
timestamp: July 19, 2014, 09:04:46 am
content: For what it's worth, AC97 CODECs run their mixers (which tend to be very good quality, with a decent number of inputs) even when the serial link isn't running. You can configure the registers with a micro, then let them get on with mixing.

author: Rasz
author_rank: Super Contributor
timestamp: July 20, 2014, 04:12:21 am
content: Quote from: Precipice on July 19, 2014, 09:04:46 am12.288MHz serial link, other than that its a great idealets see, first cirrus logic coded on digikeyhttp://www.datasheet.net/document/354444/page 14 has mixer (i refuse to register with supplyframe to make a snippet)Im sure there are dedicated audio mixer chips without digital part. simething like 74HC4067 on steroidsHarvs why exactly do you want to digitize audio? from your description it sounds like a simple case of muxing audio channels

author: Harvs
author_rank: Super Contributor
timestamp: July 20, 2014, 05:02:50 am
content: Quote from: Rasz on July 20, 2014, 04:12:21 amTraditionally that's what it's always been, a pile of analog circuits to mix everything at different levels. And it does work, using digital pots to mix each output from a combination of the inputs depending on events.But digitizing all the inputs and mixing in the digital domain does allow more flexibility. As long as there's enough processing power available it allows various filtering to be done on the different inputs. It also shouldn't be under estimated how much mixing there is to be done, off the top of my head there's at least 5 different outputs all which can be various combinations of inputs at different levels. That equals quite a few digital pots and summing amps.A look on digikey shows a stereo I2S codec is available for around $5, so that gives me 2x inputs and 2x outputs.This is a bit of a research project to see what works and what doesn't. At the end of the day nothing I've used to date has been great, so it's worth a try.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 20, 2014, 05:04:44 am
content: For a similar project, I am using a 6-channel electronic volume control with I2C interface to a low-end Arduino.And the Arduino samples the amplitude of the input audio signals and decides how to mix them according to the algorithm.This is the chip I am using: PT2258http://futurlec.com/Others/PT2258.shtml(60 cents!)It is way too much trouble to actually sample all those analog audio signals and do this in the digital domain. At least IMHO.

author: Precipice
author_rank: Frequent Contributor
timestamp: July 20, 2014, 08:42:30 am
content: Quote from: Rasz on July 20, 2014, 04:12:21 amNope, all the ones I've played with have been fine down to much slower rates, 100KHz is fine. The trigger for low power modes isn't very sensitive. If you're not pushing audio through the link, you can play really fast & loose with it.

author: IanJ
author_rank: Unknown
timestamp: July 20, 2014, 09:44:26 am
content: [quote author=Harvs link=topic=34033.msg481528#msg481528 date=1405747566There's quite a few channels, albeit it doesn't need to be anywhere near hifi:Inputs - rider mic, pillion mic, UHF, MP3 (stereo), Radio, GPS Nav, Phone, etcOutputs - rider (stereo), pillion (stereo), UHF, Phone, etcNice idea. Are you thinking about between bikes also?........just an idea.Ian.

author: Harvs
author_rank: Super Contributor
timestamp: July 20, 2014, 09:53:48 am
content: Quote from: IanJ on July 20, 2014, 09:44:26 amAbsolutely, actually between vehicles, that's what the UHF is for. It's a safety thing as well, some stretches of highway I've ridden don't have mobile phone reception for >1200kms. Having UHF along the way means you can have a chat with truckies that have come from the direction you're going in, very important info like they're out of fuel at the town you're going to can make a big difference.Another potential to is add a class 1 Bluetooth SCO link as well, but that's only for short distances. All the riders I meet on the highways use a 5W handheld UHF mounted on the bike.