There has been some discussion recently about the 4-channel cassette recorders that were used for court reporting and other logging- or court-reporter-type applications. It seems that the players only have one output and can select any combination of one or more playback channels into that one output.
This monitoring topology is actually identical to two 1-inch 40-channel reel-to-reel logging machines I have where one can listen to any combination of one through forty tracks on a single output.
The solution for the reel machine is that I have about half figured out how to create 40 different outputs–and then I have to figure how to digitize 40 channels simultaneously. All can be overcome, but the cost to do it generally terminates the inquiry.
Fortunately, there is a solution for the 4-track cassette machines: use the higher-quality 4-track machines designed for music recording. I have a Tascam 234 (as well as a 238 8-track unit). Yes, I know these operate at 3-3/4 in/s while the logging recorders are running at 15/16 in/s (normal cassettes are in the middle at 1-7/8 in/s).
What I do is record the four tracks playing at 3-3/4 in/s into the computer at 88,200 samples/second (s/s). In samplitude, after the recording is made, I make a new virtual project that has a project sample rate of 44,100 s/s. I load the tracks into that. I then adjust all four tracks to -50% speed in the object properties panel. I use resampling for highest quality. This provides a 1/4-speed playback of the original files while maintaining a 44,100 s/s output file.
The digital data, of course, is actually at some point being processed at 22,050 s/s, placing the Nyquist frequency at 11,025 kHz, for an effective bandwidth of perhaps 10 kHz.
But, that isn’t a problem as only a very few Nakamichi cassette recorders ever made better than 10 kHz at 15/16 in/s — this wasn’t even officially in the Philips standard.
So, there you have a way to migrate these recordings into 44,100 ks/s WAV files while doing the bulk of the work in 4x real time.
You may add equalization and other filtering to improve the usually poor sound after the output is at the correct speed.
I actually had to put the recordings back on this infernal format after repair of the defect (very poor recording speed due to a broken machine), so I reversed the process with the Tascam 234 without adding any equalization and the client was apparently happy (I received payment and no feedback).
As an alternate to the speed/pitch adjustment in the virtual project, one could bounce the 88,200 s/s track played at 44,100 s/s to a second 88,200 s/s track and then repeat the process of loading that as a 44,100 s/s file and it will be in time. I prefer the single-pass approach that I can do in Samplitude.
One of the things that affects my procedure is that my audio interface (RME Multiface) does not work below 44,100 s/s.
Good 4-track recorders like the 234 have not been made for a while. Find them while you can.
UPDATE 2011-11-12: I sold my broken Tascam 234 a few months ago as I have not received requests for this format in years and I have obtained two DataTape CMS-1000 professional voice logging systems. The first one I tried worked and they are modular. I didn’t even try the second one. These run from 15/32 to 7.5 in/s and have four parallel outputs. Also recommended and perhaps better at longevity as I believe the Tascams run on belts which failed.