With lockdown I’ve been going back through the archive of old tapes to digitise them. Both stereo and four track tapes. The topic comes up often on the Sound On Sound forum, and I am grateful to Hugh Robjohns and James Perrett there for the advice and discussion over the years. Along with my own practice, I’ve whittled the process down to 12 steps.
You will need a decent cassette deck. Ideally a standalone hi-fi unit with a line output. These are usually on phono connectors. You can connect this to the mic/ line input of your computer. Even better, use an audio interface so you can optimise the level.
I use a Yamaha MT4X four track. This can play regular cassettes as well as the four track ones. It’s one of the best decks Yamaha made, with a very low noise floor and a frequency response all the way up to 18kHz, out-performing the Tascam or Fostex four track machines. The only machines that are better are the fairly rare Marantz four track recorders.
The audio interface I use is a humble Focusrite Scarlett. This is a very popular model for a reason: it’s reliable, clean and neutral sounding, and best of all Focusrite have very good long term driver support.
I would advise against the Walkman sized USB players you can get (pictured above). These will be wobbly and noisy with even the best tapes, so you will not get the best out of the tapes you have.
To reduce pitch wobbles, remove the reels from the shell and put them into something really good like a Maxell XL-IIS shell. There are plugins that can remove pitch wobbles (known as wow and flutter) such as Melodyne’s Capstan, and the proprietary Plangent Processes. These are expensive.
Clean the heads of the deck. If the tapes are old and are ferric, you’ll need to do this between each tape. Wait a minute for it to dry, and just clean the metal parts. Don’t use alcohol on the rubber parts of the transport it will dissolve them eventually.
Adjust the head azimuth by listening to the playback with L and R summed to mono. Adjust for maximum treble. If the tape is really bad, boost the treble with an eq so you can hear what little high end there is. If it was a mono recording, do still record L and R, and combine them to mono to get slightly less hiss.
Even if Dolby was used, consider leaving it off. Why? Here’s the simple explanation. You can actually hear the difference for yourself- playback the tape and switch the NR in and out. Don’t get me wrong, think noise reduction is a good thing. But old tapes lose their magnetism over time, so can have reduced output level. This will cause the Dolby playback to operate incorrectly. The circuit will think it is hearing a quiet part of the tape, and apply too much noise reduction (actually it will be reducing the high frequencies, because those are the ones our ears are most sensitive to when there is noise). Professional machines let you adjust the level going in to the Dolby circuit. If that’s too technical for you, just leave the NR switched off and consider a free high end booster that you can polish with eq it later. Most of the time, you will need as much high end as you can get! For most cassettes this idea works fine, but for four track tapes it is different. These more commonly used dbx noise reduction not Dolby. This idea does not work with dbx because basically that system is more severe. Again, you can hear this for yourself if you switch it in and out as the tape plays.
Record all of the tape including the silent bits such as the blank leader. It’s much easier to edit afterwards, compared to sitting there listening to which parts of the tape you want. And keep the original file in a backup, I often find I need to go back to these later. Especially if better plugins come along that can improve the sound.
Do use noise reduction, but not on the tape deck! (This is a tip I got from James Perrett on the Sound On Sound Forum) An even better approach, is to, again, leave the NR off, but this time apply it with a plugin. The only plugin that can emulate Dolby A, B and dbx is U-he Satin. I’ve had great results with the Dolby. The tone of the tape will be much nicer because the eq curve is exactly the same as the Dolby. IIf you denoise (which I will explain in step 8) BEFORE the Dolby plugin, it does a much better job of removing the noise than Dolby can, and you get more treble back off the tape, and with a less harsh sound than no NR at all. In my experience this is especially noticeable with cymbals, and the esses in the vocals. Haven’t tried the dbx.
I mentioned denoise. This is different from Noise Reduction. NR needs to be encoded in the recording, and decoded on playback (see step 5) Noise removal uses a computer to figure out which part of the signal is noise, and which part is actual audio. I useIzotope RX. If you can find a section of tape with no audio, just noice, the plugin can “learn” the noise profile, and then intelligently remove that from the audio. I usually do a profile for each song, if there’s a run in of no audio. This way, you can deal with noise generated by the gear used to make the recording. I also add the blank leader to the profile so Izotope can remove the noise the tape deck itself adds, as well as tape hiss.
Use whatever eq you like. Don’t be afraid to use several instances in a row because you will need severe eq especially to restore the top end. I also like to use a multi-band compressor, but it’s up to you what is needed to restore the sound quality.
Often the top end is not there so you have to fake it. I find that Slate Revival is great for this. It’s a free aural exciter. The Softube Aphex gives you more control and is more effective with really poor material.
Digitise the tape at 44.1kHz 16 bit. It’s more than enough for cassettes! It will save a bit of space and processing power (you will be doing a lot of processing) compared to 24 bit or higher sample rates. Only the very best tapes can go to 18kHz, and the best cassettes have a dynamic range of around 48dB which equates to only 8 bits.
An essential post processing tool is A-1 Stereo control. It will clean up unstable stereo images caused by slight wobbling of the tape as it passes across the playback head. The “Safe Bass” feature works by filtering the bass to mono, and you can adjust the overall stereo width if you like.