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Upgrading to stereo

A project log for The ultimate vlogging mic

Recording the best headset audio in a portable form factor

lion mclionheadlion mclionhead 08/28/2018 at 05:200 Comments

For a good time, record stereo using a raspberry pi, dual USB sound cards, & a tiny 2 port USB hub. The USB hub was made of a 4 port stick, ground down to 2 ports.  Get alsa to fuse the 2 dongles into a single virtual device with a new .asoundrc file.  The 2 dongles are hw:1 & hw:2.

pcm.merge {
    type multi;
    slaves.a.pcm hw:1
    slaves.a.channels 1;
    slaves.b.pcm hw:2
    slaves.b.channels 1;
    bindings.0.slave a;
    bindings.0.channel 0;
    bindings.1.slave b;
    bindings.1.channel 0;
}

ctl.merge {
    type hw
    card 0
}


Get jackd to record from the virtual device by passing the -Cmerge option after the -dalsa option.

/root/jack2-1.9.12/build/jackd -P70 -p16 -t2000 -dalsa -Cmerge -p256 -n3 -r48000 -s &

jackd also accepts a -P option after the -dalsa option which is supposed to redirect playback to either of the 2 dongles, but this causes it to crash.


/root/jack2-1.9.12/build/jackd -P70 -p16 -t2000 -dalsa -Phw:1 -Cmerge -p256 -n3 -r48000 -s &

So there is no monitoring when recording from 2 cards.  The executable which does the magic is  https://cdn.hackaday.io/files/284961219172864/usbmic2.c

Portable stereo recording is not as easy as it was 30 years ago. The Zoom products are not as compact as a vintage recording walkman. Dongle sized sound cards only record a single channel.

The lion kingdom briefly owned a Marantz PMD-430 costing $500. It fluttered but with a belt replacement & the best tape formulation was probably better than minidisc. The lion kingdom didn't know about belt replacement & was told anything was better than tape, so traded it for minidisc, which led to years of bad lasers, cracked gears, & ATRAC artifacts.

The same Marantz still shows up on ebay for much lower prices, but there's absolutely no point in a professional tape recorder now. The raspberry pi is very slow when recording 2 soundcards, the SD card is prone to corruption, but it has wifi monitoring & 7 hours of storage.

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