Remembering the CBS Audimax

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Joined: Wed Oct 07, 2009 2:24 pm

Remembering the CBS Audimax

Post by nitefly » Mon Oct 12, 2009 10:27 am

Hey All,

I wrote this for an audio enthusiast friend who's not in radio... I thought maybe some of the old-timers here might enjoy reading it, too...


The CBS Audimax and it's sister the Volumax are magic boxes. They use extremely elegant circuits to accomplish a very difficult task in a simple and transparent way. There was a lot of research that went into them and they were ubiquitous in radio and TV all though the 60's, 70's and way into the 80's.

The main thing that has to be considered when designing an audio path for a radio station or TV station is audio level. Sure, you have to watch out for noise and distortion but the control of levels is by far the most important aspect. Why? There are two reasons: The first is the fact that a transmitter must be modulated at an exact peak level to operate legally. The other is that varying volumes from one song to another song or one program to another is very, very annoying to the listener. This also goes for the transition between music and spoken word. Imagine listening to the radio and your hearing a song you like so you turn it up a bit. When the music finishes and the DJ comes on and starts talking he's way louder than the music was. You dive for the volume and turn it down. Then, he stops talking and he plays another record. Sure enough, the audio is back down to the low volume it was before so you turn it up... How long do you think you'll listen to that station if you had to do that every time the program changes?

Prior to 1960, radio engineers didn't have a lot of help dealing with this problem. Ensuring that the transmitter would not be over driven into illegal modulation was done by putting a simple limiter in the line just before the input to the transmitter. So, if things got too loud this box would chop off the peaks. Early limiters would destroy the audio if driven very hard so they only operated in a very limited rage. Let's say 2 or 3 dB. Push them past that and they would start tearing up the audio. So they basically would only actually work when the operator had the levels up too high. Keeping levels in check was the job of the board operator and he really had to watch the meters. There were compressors around in the early days of radio but they were simple devices that would add distortion to the audio and cause a strange effect called 'pumping.'

Pumping is when an AGC circuit tends to pull up the audio level during quiet passages or pauses in the program bringing up the background noise with it. Then, the program gets loud again and the noise disappears. If the attack time on the compressor is too slow, you also get an effect where the audio is very loud right at the beginning and then unnaturally gets quoter. That's also a product of pumping.

To compound the problem, there was a trend in the 1950's where radio stations were operated with one guy in the control room. Instead of having an engineer who was there just to push the buttons and watch the levels while the talent did all the talking, the DJ ran his own board. He had to keep up with the phones and the news copy and what record he was going to play next as well as turning knobs, pushing buttons and flipping switches. Levels can get out of control real quick in that environment.

Enter CBS Laboratories. They were the first people to really tackle the problem from a systemic point of view. They considered everything in the chain that went from the studio to the speaker in your radio or TV. The challenge was to make sure that levels would be controlled effectively while maintaining audio integrity. The result of their research was the Audimax. The basic principals of the Audimax are still used today in the latest digital processors.

Here's what it does:

The Audimax works over a 20 dB range. When you put one in the program line, you calibrate it by setting it to the meters on the board. The Audimax meter has a 0 dB point in the middle with -10 and +10 dB on either side. There is a green zone in the middle that is +/- 2 dB on ether side of 0. So, if you set the levels so that 0 VU on the board's meter stays within the green zone on the Audimax, you've got it right. As long as the level stays within that 4 dB green zone, the Audimax doesn't do anything; it is in effect a unity gain device.

Now, we know that the level is not going to stay within a 4 dB range all the time so the Audimax uses a sophisticated control circuit to compensate for that by adjusting the gain to the output. Lets say our board operator is chatting on the phone to his girlfriend and not paying attention to the meters. First, he plays a record and the audio is peaking at around -6 dB on the board. The Audimax sees that this is out of the 4 dB green zone and turns up the gain. Then he plays a recorded spot and the meters are pegging at +3 dB. Once again, the audio is out of the green zone and the Audimax turns the gain down. The transmitter is kept within legal modulation limits and the listener doesn't hear the gain differences on her radio. However, the station's engineer is sitting in the shop where the Audimax is and he's watching the meter while all of this is happening so he strolls down to the studio and yells at the DJ about not watching his levels. Our DJ friend has to tell his girlfriend goodbye until the engineer leaves.

The Audimax does all all of this gain riding transparently without distortion or any pumping because it is intelligent. Unlike a simple compressor, it keeps tabs on what the audio is doing and figures out the best way to deal with gain changes. The gain controller, also known as a Voltage Controlled Amplifier (VCA), takes in input from three different monitoring circuits to determine what it should do next.

The first circuit is the level detector. This is the part of the device that figures out what the incoming levels are and sends a control signal to the VCA. The level detector in an Audimax tries to figure out what the average level of the audio is at any given time and keeps the VCA at a gain where the output will sound as much like the input as possible but at the proper level. Hoover, if there is a sudden gain change, the detector sends a signal to the VCA to react quickly enough to change the gain before your ears will notice a difference. This is called platform limiting and it is very natural sounding compared to a compressor that is always hunting for the right gain.

The next circuit the VCA gets instructions from is the gate. The gate is basically a traffic cop for the VCA that tells it when it is OK to make gain changes without causing pumping in the output audio. During short pauses in the program, the level detector is tempted to tell the VCA to raise the gain all the way up. If it were allowed to do that, you'd hear the background noise coming up and then when the audio returned, the VCA would have to suddenly turn the gain down and the attack would sound unnatural. The gate watches input levels and if they drop below about -14 dB, it freezes the VCA at whatever gain it is operating at until the audio returns. This way, the 'hole' sounds natural and the listener never hears anything that would indicate that the audio is being processed.

The last circuit the gain controller gets instructions from in an Audimax is the RTZ circuit. RTZ stands for Return To Zero. Let's revisit our lazy DJ again, he's playing a taped show and decides that he's going to go down to the station's kitchen and fix a snack. While he's there, he gets to talking to the receptionist and looses all track of time. The show finishes up and the tape runs out. Dead air. There is no audio going into the Audimx and it needs to diced what to do. The RTZ circuit is driven by a memory unit that keeps track of all the gain changes over the last 10 seconds. It works with the gate circuit. If the gate kicks on and freezes the VCA, the RTZ's memory unit starts watching the gate to see if it does anything. If the gate stays on for more than ten seconds, the RTZ takes over and initiates a very slow gain change that resets the Aduimax to unity gain. Lets say that the taped program was averaging -4 dB when the jock left the studio. The show ends and the gate kicks in and freezes the Audimax gain at -4. Since he's not paying any attention to what's on the air, more than ten seconds goes by and the RTZ circuit kicks in. The meter on the front of the Audimax begins to very slowly move toward zero and the RTZ holds the gain there. The Audimax is now in a standby mode acting as a unity gain device until it's fed more audio.

Meanwhile, the engineer sees the RTZ function kick in and goes looking for the DJ. Having now had as much of the DJ's carp as he can handle for one day, he finds him in the kitchen talking to the receptionist and grabs him by the hair, he then drags him down the hall to the studio while letting everyone in the building know that if the poor DJ pulls one more stunt like this, he'll be unemployed. After picking himself up off of the floor, the DJ grabs a cart with s commercial on it and plays it on the air. The audio hits the Audimax which has been sitting at unity gain since the RTZ circuit took control several minutes ago. Since the audio is peaking at +1 when it hits, it is within the green zone and the Audimax makes no gain change but is now fully functional again because the RTZ circuit has switched off and the level detector and gate circuits are once again in control of the VCA.

So, you now see that the Audimax is and incredibly intelligent box that deals with levels so cleverly that you probably never noticed it working. After almost 50 years, it still can hold it's own in a an audio chain.



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Re: Remembering the CBS Audimax

Post by cboone » Mon Oct 19, 2009 5:31 pm

Ahhh the good old Audimax and Volumax.....We had them at 560KLVI in the a Top40AM station, they were mandatory! We ran a Audimax on the STL....and then at the transmitter site,a Volumax, a 5 band graphic eq (for AM?? yep!) and then the BL40 Modulimiter audio processor...

The crappy CCA never sounded better! :shock: though it rarely like the asymetical modulation.....ran through 4-1000 modulation tubes like crazy!

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Joined: Wed Oct 07, 2009 2:24 pm

Re: Remembering the CBS Audimax

Post by nitefly » Tue Oct 20, 2009 7:43 am

I saw one of those Modumlimiters on eBay the other day...They were a nice box to put between the two CBS units. Love that sound. :) JC

Posts: 5
Joined: Wed Oct 07, 2009 2:24 pm

Re: Remembering the CBS Audimax

Post by nitefly » Sat May 28, 2022 1:05 am

This is still here? Wow! LOL

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