Why did late 60's/early 70's rock singers have two mics taped together?

I've been enjoying "Guitar Heroes at the BBC" on BBC4 where they compile clips from Whistle Test, Rock goes to College, TOTP etc. I've always wondered why rock singers from a period of only a few years would have two mics taped together. By the time I was paying attention in the late 70's the practice seemed to have stopped so I suppose it was a technical development that made the change.
I asked the question on Twitter and Facebook and got great rock'n'roll answers; "...so they could take it to 11", "early form of stereo recording" etc. In fact when I went back over my old BBC notes I had been told why they did it but only a few weeks out of university I don't think I understood common mode rejection!

Ronnie Van Zant of Lynyrd Skynyrd c.1973 - two mics!

So - having re-read my notes and had a trawl around the web (my word, there is some awful rot spoken by people who know very little!) here are the two reasons (and I'll list them based on the technology that fixed the problem), they both rely on the fact that the two mics are wired anti-phase to each other and the assumption is the singer sings predominately into only one of them (doesn't matter which).

1. pre-compressor/limiters you needed a way of loosing some of the induced stage and line noise - this does it.

2. pre-parametric eq - you needed a way to reject howl-round and this does it.

So - you mix the anti-phase feeds in two channels on the desk and all noise/feedback etc gets canceled and the voice (predominantly coming down one feed) remains. Interestingly another technique to gate a mic is to have either an optical detector on the mic stand or a pressure mat in front of the mic which mutes the channel when nobody is near the microphone.

Loudness Measurement

This how to guide highlights the benefits of the Loudness Measurements software on the WFM6000/7000 series and WVR6000/7000 series products.
Until NAB this year (when Tek introduced this update) the predominant way to measure perceived audio loudness was with some propriety instrument like the Chromatec (approved by Channel Four). Tek have done the right thing and not only followed Ofcomm's new guidelines but they've devised a scale that is as easy to read/understand as a PPM.

Hopefully they'll do the same thing with PSE measurement and put Harding out of business!

Balanced to unbalanced audio - wiring?

Simon, one of our wiremen asked me the following question;

...why do we always cut-back the black core when wiring FST into RCA (phono) connectors?

Back when all broadcast gear drove balanced lines with a proper rep-coil (a 1:1 audio transformer) you could safely bodge-unbalance a balanced line by shorting the cold to the screen and you get full signal across half of the sending rep-coil and everything worked. Nowadays not all equipment drives a balanced line this way - may bits of gear use op-amps to derived the +ve and -ve going halves of the balanced pair (via the inverting and non-inverting inputs - think 741 Op-amp). If you pull one of the signal cores to ground you effectively short one bit of silicon and it sits there warming up. That may not be a problem, but in the case of Avitel distribution amplifiers (well, after 1995) they driver stage burns out after a few months (long after the SI engineers have left!). This exact problem bit my backside when I worked at Oasis TV and all the monitoring switchers in the machine room had unbalanced inputs.
The only downside of using my method is that you loose 6dBs of level, but probably into a piece of equipment that wasn't calibrated (like a DVD recorder or TV). I think that's a worthwhile compromise to avoid the possibility of frying the backplane on audio distribution amps (or suchlike).

Cheeky cheeky!

I've just been finishing off a studio build at The Law College - I chose a Yamaha 01v96 as the audio mixer - it has loads of ins and outs and very controllable. They didn't want to go for a full-blown talkback so I used a couple of ins for the director's open and switched mics and a couple of the spare Aux'es to feed the wallbox foldback feeds (the interviewer's earpiece and the floor foldback wedge). For the director's desk I had the metalwork dept at Bryant make me a panel with a mic on it and talkback key so that the director can key the mic and talk to the whole studio over the floor foldback but the interviewer's earpiece always gets the feed. You could also send a mix of the playback out of the wedge so that the studio crew get in the mood(!).
Anyhow - a combination of a mic that wasn't very sensitive and the general noise in the control room meant that the earpiece feed was a bit noisy and possible distracting to the interviewer.
The solution - the 01v96 has a noise gate and compressor per main input channel - I stuck a gate set to -50dB on each mic input (the open and switched feeds) and all was good. On one hand it seemed a bit naughty to use a noise gate but on the other hand it worked brilliantly!

Sorry to not be blogging so much at the moment - I'm working late most nights to stay on top of my day-job.

Passive volume controls

The PDF in the link is a good explanation of passive volume control for audio circuits and how to alter the law of potentiometers.

$13,000 speaker cable!

Save us from Audiophiles!

John Dunlavy, who manufactures audiophile loudspeakers and wire to go with it, does think questioning is valid. A musician and engineer, Mr. Dunlavy said as an academic exercise he used principles of physics relating to transmission line and network theory to produce a high-end cable. "People ask if they will hear a difference, and I tell them no," he said.
Mr. Dunlavy has often gathered audio critics in his Colorado Springs lab for a demonstration.
"What we do is kind of dirty and stinky," he said. "We say we are starting with a 12 WAG zip cord, and we position a technician behind each speaker to change the cables out."
The technicians hold up fancy-looking cables before they disappear behind the speakers. The critics debate the sound characteristics of each wire.
"They describe huge changes and they say, 'Oh my God, John, tell me you can hear that difference,'" Mr. Dunlavy said. The trick is the technicians never actually change the cables, he said, adding, "It's the placebo effect."

I remember BBC Research Department did a similair set of tests in the late eighties and concluded that the best paid ears in the industry (with the best amps and speakers) couldn't tell the difference between 10A mains cable and very expensive OFC cable. Around the same time Practical Electronics magazine did a blind test with the editors of What Hifi and HiFi World and they concluded that the mains cable they'd taken off a lawn-mower was the best sounding cable!

With all this in mind I'd point you at previous posts;
Audiophile at I-Like-Jam and Why do people have to have their say?