Getting on top of it

I finally received a welcome email on Wednesday. After a plea for help with the MBI mixer on a radio forum, someone put me in touch with someone who really knows about the workings of the mixer, and was able to explain to me how to connect up the telco channels, and explain what some of the other mystery controls were. So, last night, I took a pile of patch cables, and tried connecting up the channels as described. Ok, I had to think about where to get a prefade presenter mic feed from (ended up taking it from an aux output), but it all wired up ok. There seems to be some level fluctuation which I think it TBU related, and there's some noticeable feedback on the line, which I think might be due to a malaligned cleanfeed mix-minus circuit... that means I'll have to take the channels out either tonight or tomorrow to have a look at them.

I then went mad. I've long wanted to swap the distribution amp and compressor over in the audio chain, so the compressor comes first, and the DA provides 4 compressed outputs, rather than compressing one of the DA's outputs. Traditionally, we've fed an uncompressed feed to the transmitter, but having been listening to Priory FM this week, and hearing a presenter pop the mic I though I'd better reduce the mixmum level of spikes, because in the tx audio chain, there's an AGC, which has fast attack and slow release. The mixer will kick out about 20dBU maximum, so if there's a very bad pop, the AGC will take 10ms to knock off 20dB, and then recover over the following 3 seconds... which quite frankly sounds crap.

Now I used the SCOM as a soft limiter, with a 2:1 compression ratio above -4dBU (soft elbow). So what would come out of the SCOM in a "pop" situation? Well there's +20dBU out of the mixer, 24dB above the elbow, with 2:1 compression, divide by 2 to get 12dB, and add that to the elbow at -4dBU, giving max output of 8dBU. Now, as it happens, I've get the digital chain (logging and web stream) to record 0dBU as -12dBFS, giving 4dB headroom. The digital part of the chain used to induce a bit of distortion at higher levels, but I've tweaked the input and output levels to reduce this, so that everything, right through to the MD record, which is at the end of the digital chain, sounds lovely.

So what of the remaining desk faults? Well, there's come noise on the aux 1 and 2 post-fade outputs of the guest 1 mic channel. I have no idea what's causing it. It doesn't appear on the prefade signal, or on aux 3 post-fade. The interpatch lines between the talkback circuits need to be checked - I've cobbled it together for now, but I might need to set some jumpers on the telco boards. The split TB doesn't appear to work, I'll have to look into this, it might be an interpatch problem. I've still got a problem when replacing bulbs in the TB switches, as the desk locks itself into "dim" mode - looks like a short or some other fault which will require tracing. The phantom PSU is fixable, if I can be bothered. I've given up on the IT12 unit completely. We'd need 2 for it to be useful anyway.

I'll have a quick look tonight, but I'm training Mark at 8...