Alban’s blog

For 4 years, Radio Esperanzah! capture about 30 hours of live each year for the Esperanzah! festival. Until the 2007 edition, we used various recorders (from Archos Gemini to Tascam Compact Flash recorder). But recording something like 20 concerts with a mobile device consumes a lot of time. Lives at Esperanzah! are performed sequentially on two stages. You need to plug and unplug everything 80 minutes and move the capture system to the other stage. It’s painful.

For this 2007 edition, we betted on a new system : install a computer on each stage for the whole festival. The idea is simple : each “satellite” records continuously and recorded files are retrieved periodically by our NAS server via wifi. No (or less) useless movement, the same setup installed for the three days of festival, no cable to plug/unplug, …

A great idea … but it required a lot of preparation. Here are some of the problems we had to solve.

Hardware selection

The first job was to choose and find the hardware. The base system is an old motherboard, a little RAM and an old but safe hard disk. Two of our old workstations have been recycled in this goal. One of them is dead few days before the festival but Bastien saved us (one more time) with a small station.

After the base system, we need two special devices : a sound card and a wifi card.

For the sound card, our needs were simple : 2 or 4 balanced analog inputs and linux support. Our final sound card selection was a Terratec Phase 22 and a M-Audio Delta 44. As the RTBF radio team agreed to provide us their ambient microphone signals, we installed M-Audio Delta 44 cards.

For the wifi cards, we bought D-Link DWL-G510. Even if the rt61 module installation was a story, these cards worked perfectly.

Recording level

After a email discussion with the sound engineers of Arto, it was clear than nobody could guarantee the level of the output signal for each festival concert. We had to find a solution to make the recording level as good as possible.

We used our Behringer MDX4600 and a MDX4400 provided by 48fm to limit the 4 output signals. The 4 small IO meters helped us a lot to control the signals.

We didn’t experience clipping problems during the festival .. except when Radio Bemba started playing .. The five first minutes, all the output levels were, may be, 10dB louder than the previous concert. The resulted sound is a bit … compressed. And all radio listeners followed the compressor settings changes in live during 3/4 minutes :-/

Recording in loop with alsa

The hardware problems solved, the matter was how recording in loop during 3 days. It could look like our recording of broadcast system .. but using jackd didn’t seem to be a good idea.

We had to find a alsa recorder which can loop and make small files (about 5 or 10 minutes). I thought about using record from xawtv-tools as I already did. Unfortunately this small application doesn’t support more than a .. stereo signal.

By making some tests with a simple arecord .. I tested a arecord - | split just to see if it could work (split is a base unix tool which writes the standard input into files of given size). After two/three tests, it began to look like a real solution .. It doesn’t seem very sophisticate, but it really work fine. arecord outputs our 4 channels in raw format, split writes in files. The only requirement is choosing a file size which doesn’t split the 4 channels at the end of file. One of the limitations is that the split file naming strategy is very poor (.001, .002, .003, …).

So we finally recorded about 20 lives during three days via arecord, split and a simple unix pipe. This daemon recorder is started by a init.d script and fills a 10Gb partition with 5 minutes files.

Compress and retrieve

Recording in loop with a M-Audio 44 is great .. but it fills quickly a partition These nice raw files weren’t our final objective. We needed to retrieve this recording via a wifi connection. No way to retrieve it in raw format. So a small cron script created ogg files (in high quality) in another partition. A simple rsync daemon allows our NAS server to retrieve files. As recorded files are only 5 minutes long, the NAS server retrieves the recording with a delay of only few minutes.

Few troubles

We can say that the capture system worked fine. But as nothing is magic, we experienced some troubles :

Installing the two satellites was a fight against the time. We had only two hours to install, plug and test “on site�? the two systems. The first day, we have to spend a lot of time on the two stages to check the levels and the whole system.

The wifi network was unreachable on one of the stages. We had to go and retrieve the files with a laptop.

A arecord process died the first day. The daemon script was too naive and the nagios supervision wasn’t set for this processus. We discovered the problem after two hours. We lost one and half concerts

The second day, the compression cron was buggy .. it compressed the same files every 10 minutes. We fixed the problem on the first satellite, but the second satellite was unreachable. Between two concerts, we connected the machine to find a load of … 53 ! But the arecord didn’t stop ! Yes, arecord can record under a load of … 53 ..

In fact, mixing the 4 channels live files was a heavy job. Even with an heavy workstation (AMDx2, stripped SATA disks) and Sophie’s patience, it was a painful and repetitive job.

Evolutions

The record level could be managed better than currently. Most of the concerts have been captured with a peak level at -20db … I had prepared a small tool to reproduce the behaviour of xawtv-tool record (which turns down the mixer when the output level peaks), but we didn’t use it.

We thought about a alsa/jack recorder tool with : splitted output file support, level control, daemon mode, etc … To be followed …

Tags: alsa, esperanzah, live, loop, m-audio, record, rsync, wifi

This entry was posted on Tuesday, September 4th, 2007 at 7:25 pm and is filed under radio. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.