Audio hardware experiences

Mostly, but not exclusively, about how these gadgets play along with Linux.

E-Mu 0404 USB audio interface

Mostly works with the standard snd-usb-audio module, but it is a little quirky:

• In alsamixer, there is a sampling rate selector with no indication about real values. The lowest one is 44.1 kHz, then 48, and so on. Decent (but not all) software can access this properly, as the mapping is done by the kernel module. But you can find some interesting glitches when changing it manually, and in some cases you need to change it to the correct value.
• The only sample format for plain audio is s24le. Some software must be manually set to output this.
• Device 1 (alsa terminology for the second device on the same card) only accepts s16le samples, 44.1 or 48 kHz. It turns out this is only meant for AC3/DTS passthrough from movie players to external decoders. For this, the samplerate must be set manually, mostly to 48 kHz (shown as 20% in alsamixer) .
• Raw audio via SPDIF is missing. The SPDIF output cannot be unmuted via alsamixer, though there is clearly some signaling indicated by my decoder. Likewise, SPDIF inputs stay muted, even if there is some sync LED activity.
• The direct monitor volume controller is a digital one, not a broken pot as some people might think ;)

One workaround for the lack of raw SPDIF is the AC3 encoder plugin. Of course, it comes at a loss of quality, and does not work with higher bitrates.

Having to use a dedicated power supply is a minor inconvenience, but it has some surprising benefits: you can use the E-Mu as a simple standalone mixer or AD/DA converter. (SPDIF in/out does work with direct monitoring.)

The E-Mu should generally be powered before hooking up the USB. Sometimes it works the other way, but not always.

Zoom H4n portable recorder / USB audio interface

Works with Linux as a standard USB audio device. Bitrates are rather limited compared to recorder use, but this is a device limitation; it must be chosen when connected.

As a standalone recorder, this has turned out very nice. The user interface is pretty logical, easily accessible by the hand holding it. I could use a lower-latency playback for theatre use, but this is understandably not designed for it; playback is there mainly for quickly checking what you recorded.

Tascam US-122 USB audio interface (sold)

There is a special snd-usb-usx2y driver in vanilla kernel. It also needs firmware and loaders; set usb-usx2y in ALSA_CARDS to get these in Gentoo, and emerge alsa-firmware and alsa-tools. There used to be udev scripts to get it working upon plugging in, but after the endless overhauls of the udev/init system, I generally use a script. You may have to adjust the nrpacks value, it usually works without any setting.

The nice thing about this Tascam is that it has no software mixer. Once you have it up and running, there are no hidden features that might depend on a closed-OS driver. Everything is accessed with real pots and switches, and there is a solid, almost professional feel. It has turned out very reliable in productions on and off stage, once you have set it up.

Unfortunately, it is a little picky about computers and their USB implementations. I have used it succesfully with Linux on a number of x86, x86-64 and PowerPC machines, but a Thinkpad has failed. I presume it is because the machine has no real UHCI (USB 1.1) and the EHCI has to emulate it, poorly. There are other cases where this card fails, so try before you buy. The nrpacks option might help, but in the case of my Thinkpad, it only changed it from not working at all to very glitchy.

Other links

• Linux audio tips

---

Risto A. Paju