So You Wanna Do 96k!
By Roger Nichols

44.1kHz is for wimps, you say? Real men mix to 96kHz/24-bits, or 192kHz/24-bit, or maybe even DSD (Direct Stream Digital). More and more 96kHz equipment is becoming available, from converters to recorders. But figuring out what works with what makes the phrase “Would you like fries with that?” seems like a less distant alternative.

Two-Hole, Or Not Two-Hole

When 96k first reared its ugly head, there was no standard for getting the digital audio stream from one piece of equipment to another. The use of an XLR terminated AES cable to carry the double speed bit stream was going to be pushing the limits. The smart guys wanted to use coax cable designed for the new frequency requirements. Coax cable would easily carry 192kHz/24-bit signals as well. Somewhere along the line the idea was dropped because coax doesn’t look like it is carrying audio, but a nice XLR connector is familiar to audio facilities. This is the same mistake that was made the first time around.

So now the XLR digital cable is the data carrier. Each piece of digital equipment has an interface chip that turns the digital audio bit stream into a signal that is designed to withstand the hostile environment of a length of wire. Similar to a normal RS-422 serial connection, the driver and receiver chips are robust enough to work over varying cable lengths. The current implementation, however, would not handle the double speed data.

Some manufacturers decided to split up the signal and send it out a pair of AES connections. This is called the “Double Wide” format. Normally one AES cable carries the left and right channel of a 44.1kHz or 48kHz signal in one bit stream. (There is already room for 24-bits in AES data) The receiving equipment splits the data back up into the left and right components and sends it on its way. Two AES cables carry twice as much data; just what is needed for a 96kHz signal. Each audio channel of the 96kHz signal is split in half. The odd numbered samples are sent on the AES port’s left bit stream, and even numbered samples are sent on the right bit stream. Each AES cable is thus carrying all of the data necessary for one audio channel at 96kHz/24-bits. Apogee uses the two-hole interface for their PSX-100, 96kHz/24-bit converters. Genex uses the two-hole interface for the GX-8000 MO recorder. The Genex GX-8500 supports both one-hole and two-hole formats (as well as four-hole for 192kHz/24-bit).

Some manufacturers decided that one hole was enough. In the beginning it took expensive handpicked driver chips to spit out the double speed data stream. Now chip manufacturers are making faster drivers that are designed to work with the high speed data, so extra cost in manufacturing is no longer a factor. This format is known as the “Double Fast” format. One reason for deciding on the one-hole approach was that interconnection would be the same as for existing AES hookups. One cable is all you need to get from device “A” to device “B”. The drawbacks are that you need to record on a 96kHz/one-hole recorder, and you must be very careful with cable selection and length. The new Alesis MasterLink ML-9600 hard disk/ CD-R recorder is a one-hole machine. TC-Electronics’ new Finalizer-96k is also a one-hole box.

The kicker is that very few devices give you a choice between one-hole and two-hole interconnects. This means that I cannot use my Apogee PSX-100 converters to feed my TC-Finalizer-96k. I could use the 96kHz/24-bit converters built-in to the Finalizer-96k, (which are great, by the way), but then I can’t get into the Genex GX-8000, or into my Pro Tools. What’s a guy to do? Nobody makes a one-hole to two-hole converter box.

Let’s Split!

This two-wire method works very well when trying to record 96kHz data on existing 48kHz/24-bit recorders. You just use four tracks to record on and everything is perfect. I record 96kHz information into four channels of my Pro Tools system. It works great. The Apogee PSX-100 has additional bit splitting capabilities for multi-track recorders that can only handle 16-bit data. There is an Alesis ADAT I/O, and a Tascam TDIF I/O located on the back panel. For 96kHz/24-bit recordings all eight tracks are used for the various pieces of the split up signal. When you play back the eight tracks of data all of the pieces are put back together and sent to the D/A converter as 96kHz/24-bit information.

If you have an 16-bit eight track or better hard disk recording system that supports the ADAT light pipe, you could record the 96kHz/24-bit audio and play it back with no problems. You should be able to do hard edits to all eight tracks, but fades or level changes will screw up the data and rip your speakers right off of the wall when you play back the resulting digital audio train wreck. In a 24-bit Pro Tools system with the 96kHz/24-bit data stored as a four-track stream, you can edit, perform fades and change levels as long as you do it to both halves of each channel at the same time. Make sure the tracks are grouped together so that everything gets edited together. Did I say that there would be big trouble if the tracks were not locked together?

The 96kHz Alligator

Ok, so you now are riding on the bleeding edge of technology. You are even transferring your cassettes to 96kHz/24-bit. Life is good… almost. With an upper frequency response limit of 40kHz you can make excellent dog whistle recordings. I’m just finishing up my first DVD entitled “Studies in Parallel Harmonies Above 20kHz”. You could play it to entertain your pets without disturbing any humans. Another project idea is to record everything an octave below where I really want it, then pitch shift every instrument up an octave so I will have plenty of supersonic overtones.

Even if you recorded everything at 48kHz on your ADATs, the act of mixing, EQing and adding reverb and effects will generate program content above 20kHz. That is the good news. The bad news is that you can’t check to see what is up there because you can’t hear it. You can look at it with a spectrum analyzer, or zoom in and look at the waveform, but you can’t really tell by looking, what effect it will have on your hearing.

Audio equipment is designed for a fairly flat response from 20Hz to 20kHz. We have known for a long time that there are problems with recording information below the 20Hz limit. DC components must be filtered out, capacitor noise and power supply ripple must be eliminated, and does anybody remember “turntable rumble?”

With 96kHz recordings I have run into some supersonic problems that you should watch out for. Remember I said that supersonic material is generated during the mixing process. Sometimes these can be pretty healthy transients generated by the music. Other times they can be harmonic impulses that are caused by the console EQ. In the 20Hz to 20kHz world there is no problem because if something causes a click you usually hear it and fix it. Supersonic transients go undetected. I have some mixes that contain some of these supersonic transients. When I play back the mix in the studio, or on my studio quality gear at home, everything is fine. When I play it back through a consumer power amp and speakers there is a giant click. When I listen on headphones powered by very expensive amp, everything is fine, but when I plug the headphones into a $500 receiver, the click made my nose bleed.

Low price amplifiers contain circuits that can not change the voltage fast enough for the high frequencies coming from the 96kHz material. It is kind of like the click you hear when you have the bass turned up too loud and your speakers hit the stops. (My daughter Ashlee actually likes the extra click added to the kick drum attack. Oh well, she’s out of my will.)

The Answer.

I talked to one mastering engineer about the problem and he said he just rolls off the stuff above about 22kHz so he won’t have that problem, and as long as the end product says 96kHz, who will know? Wait a minute. Doesn’t this negate the need for 96kHz? Have I wasted all of my money again?

I guess a good comparison would be owning a Dodge Viper. You can’t find many places to drive 180mph, but it sure impresses people who see it in your driveway. I guess the Apogees and Genex and Mytek and Alesis and TC 96k stuff looks good in my rack, so maybe everything will turn out ok. I’ll have to think about this and get back to you.