Last time I talked about used equipment. I left out the part about the used ADAT market that is starting to emerge. They used to be scarce as hen’s teeth, but after the introduction of the ADAT-XT, the “vintage” ADATs started showing up in the want ads.

The reason I waited until this month is because I happen to be sitting in front of a stack of four ADAT M-20 20 bit machines I’m using for a Bela Fleck tracking session. First impression says that after the introduction of the M-20, vintage ADATs will flood the used market. This could be good news for those of us who want to ad 24 bit recorders to our ADAT stack. Yep, on top of these four 20 bit machines I have two vintage (digital equipment more than two years old)ADATs recording eight channels of 24 bit audio.

Where am I getting this 24 bit signal to record, you ask? From a pair of Apogee AD-8000 24 bit analog to digital converters, I answer. (I actually only needed one, but two looks better in the stack and it makes everyone think I am serious about it.) From the 24 bit digital out of the Apogee I feed a pair of Rane PaqRats. The PaqRats split each pair of 24 bit signals into four channels of 16 bit information so that it can be recorded on 16 bit machines. Luckily I just happen to have these 16 bit machines stacked on top of the four M-20s and the two Apogee AD-8000s.

The Apogee AD-8000 has option slots in the back that will accept future AMBUS cards. One card will interface directly to Pro Tools so that you can use the AD-8000 instead of the Digidesign 888 box. Others will connect directly to DA-88 machines, or to ADAT machines. The ADAT interface will have the PaqRat software so that you can perform the bit split recording. These interfaces were not yet available, so I used the actual PaqRat boxes.

Bit Off More Than You Could Eschew?

At this point I would like to clear up some bit concerns. It is clear that more bits and higher sample rates are better. It is also clear that many people aren’t always hearing what they think they are hearing.

There is a difference between the resolution of the recording storage medium and the resolution of the converters on the record and playback side. As an example, many DAT machines advertise 20 bit converters for A/D and D/A, but the DAT tape only records 16 bits of the information. If the 20 bit converters sound better, it is because they are doing a better job of producing the resulting 16 bit information than the straight 16 bit converters did. A Turbo Porsche doesn’t work as hard to go 100 miles per hour as a Geo does. It is the same kind of deal. I have had DATs sent to me with notes saying they were 20 bit DATs. Nope, sorry.

The M-20 ADATs were announced about the same time as the introduction of the Tascam DA-98. Even sales reps at trade shows told me that the DA-98 was 20 bit. It does not record 20 bit, but it does use 20 bit converters. Big difference.

The same thing has happened to me when using the Yamaha 02R. All of the digital ins and outs are full 24 bits. You can change the output resolution to be 16 bit, 20 bit, and 24 bit. It is very easy to hear the difference between 16 bit out and 20 bit out when you have hi bit audio flowing in to the console digitally. When I finally had some 24 bit audio to play with, I kept switching the output between 20 bit and 24 bit. I didn’t hear any difference. I then realized that the 02R control room monitor output only has 20 bit 8x oversampling converters. My Apogee DA 100 was connected to the digital out, but they are only 20 bit. Just because the switch says 24 bit doesn’t mean that is what you are going to hear without the proper converters. Basically I had this 24 bit Apogee stack, and I had no way of telling if it was any better, because I couldn’t play it back. Until the 24 bit D/A showed up. Then it was amazing. It is true, once you’ve heard the 24 bit stack, you can’t go back.

Last Little Bit

Another thing that comes up quite a bit are questions about the difference technically between 16 bits and 20/24 bit stuff. The general consensus is that the difference between 16 bit and 20 bit is just the dynamic range and noise floor. 16 bits has 96 dB, 20 bit has 120 dB, and 24 bit has 144 dB. If you are recording rock music or something else that is mostly loud, then you don’t need the extra bits. Not true. Here is the way it works.

Lets say you record some really loud music with big bass, big sounding drums, blasting horns and whatever else you dig up. Now record it into any digital audio editing program and zoom way in to the bit level. Now pretend that you can zoom in until the difference between one level and one bit louder is one inch up on the graph and one bit lower in level is one inch down.

This is your step size for 16 bit. If the guitar level for that sample was half way in between, the converter could only give you a value that was either one inch up or one inch down. If you were recording 20 bit there would be 16 level choices between each inch mark. The accuracy is within a 16th of an inch. If you were recording 24 bits, there would be 256 steps between each inch boundary.

So, the resolution vertically is improved which makes the sound reproduction quality better. In the other direction, left to right on the graph, if you increase the sample rate to 96kHz, you are only doubling the resolution, but 24 bits increasing the resolution by 256 times. Cool, huh!

Next month, the quiz.