Alban’s blog

The afternoon was dedicated to improve the udp link between our studio workstation and our streamer server. We’re using jack.udp since 2004 to transport sound over network.

Using jack.udp to link two jackd servers creates always an unsolvable problem : the two jackd servers aren’t synchronized. They are synchronized on their own sound card … There are always to many or not enough frames sent by the first jackd for the second one. When a buffer underflow occurs (our case between these two servers), the jack.udp lets the data into the buffer and sends to the jackd server a full period of .. zero. In pratice, we’re using a small period in the receiver jackd server, so the sound “holes” aren’t really audible. It is not the perfect solution, but we’re making web radios .. not an album mastering.

We should test netjack in a near future. It provides a real solution for this use case. But the startup dependencies between the two jackd servers could be a blocker problem.

I made two patchs for jack.udp today :

The first patch manages the support of a configurable jackname (and not the default "jack.udp") We’re using several jack.udp instances on the studio workstations and the patchbay rules can be written without known names.

The second patch fixes the support of packet index. Each jack.udp packet contains an index. But the current code sends bad indexed packets and doesn’t make an effective index check. I fixed these bugs and made a index check which only logs a message. The previous code was killing the process :-/

The debian unstable package of this patched jack.udp is available (amd64 for etch).

A small idea : the jack.udp daemon starts on our streamer server is piped with logger to log stderr jack.udp messages to syslog (2>&1 | logger -i -t jack.udp). It makes things really simple to monitor this transport critical for us.