Crash Course in Analog Tape Machines
Source: Mix Magazine
What if you were enlisted to assist on an analog recording session? One of my students is headed out of state to do just that. It’s been a year since we’d ‘analog’d’ together, so I figured he needed a few mental refreshing bullets, so here goes…
- Make friends with staff
- LEARN HOW TO THREAD THE MACHINE ASAP
- STAFF will likely do the alignments, but you will want be sure to query the engineer as to the preferred Operating Level, Tape Speed and EQ (details below).
- Take every opportunity to observe when geeks are melding with machine.
- ALWAYS put the machine in SAFE MODE after a take until you are sure it is time to record again.
DAILY MACHINE MAINTENANCE
Heads should be cleaned throughout the session especially at the beginning, so you get a feel for the tape’s condition – even new outta-da-box tape may shed – some more than others – this kills High Frequency (HF) response. Use 99% alcohol or denatured alcohol. Dedicated cleaner may be extremely potent / caustic, so keep far away from plastics (like VU meters).
LOOK closely and you’ll see a technique I use to make oxide buildup – and head wear – visible. I lay a sheet of white paper on the deck plate in front of the headstack. Good, non-glare, overhead light reflects on the paper to the head.
Do not use alcohol on the pinch roller – either a dedicated rubber cleaner or diluted windex / formula 409 on a cloth / paper towel is better. Be wary of pinch roller degradation warning signs – when the ‘puck’ transitions from firm to soft / tacky / gummy.
Be super aware of hidden magnets and their proximity to tape and machine – Laptops, speakers / monitors / tools…
- Tape obviously has no time stamp so keep really accurate notes.
- Set 00:00 at the head (beginning) of the tape. ,
- tapes are stored PLAYED = Tail Out
- Label the reel, label the box, and if you’re handwriting sucks, use your laptop for session logs and make sure to get access to the local printer.
- Tape machines have Erase, Record (sync) and Playback (repro) heads. -’Print’ tones at the head – one minute (minimum) each: 1kHz, 10kHz, 15kHz, 100Hz, 50Hz.
- Insert Leader tape after the tones.
- Waver of more than 1dB @ 15kHz is not an encouraging sign.
- In SYNC mode, The machine will automatically switch to input when recording just like PT!
- Obviously the remote let’s you choose monitor modes – INPUT, SYNC (for overdubs) and REPRO for mixing.
I pretty much use 15 inches per second because it is both economical (a 10-inch reel lasts 1/2 hour) and good for rock because the low frequency response is extended compared to 30 IPS, a speed that does not allow you to push low end, especially kick drum. At either speed, I do not recommend EQ-ing (boosting) low end to tape – esp @ 30IPS – low end boost is better to do on playback.
The first image in this link shows how tape speed affects frequency response.
Standard operating levels are expressed two ways: in nanoWebers per meter (185nWb/m, 250nWb/m, 320nWb/m or 520nWb/m) or in the amount of dB above standard operating level, as ‘plus 3, plus 6 or plus 9.’ For example 250nWb/m is +3 above 185nWb/m. Follow this link for details…
If, for example the test tape is @ 250nWb/m, but the eng wants to run 3dB higher (approx 320nWb/m), set the test tape ref tones for REPRO and SYNCH at -3dB VU.
- BEFORE record alignment, a known +4dBu reference tone is sent to all channels.
- Machine is switched to input and INPUT CAL is adjusted for 0VU.
- DURING Record alignment, machine is switched to REPRO. Adjust REC CAL / Level for 0VU.
- Record alignment begins with bias – read article above as this should be done by staff – then INPUT cal, record level and finally by HF record level..
- BIAS is primarily to minimize low frequency distortion and maximize punch. BUT, Bias affect High frequency response as well.
- Bias is typically adjusted for a peak @ 10kHz and then increased (CW) until 10kHz level falls ‘x’dB – the amount will be specified by the machine and / or tape manufacturer. This method is called ‘overbias’ and a few dB of HF are sacrificed in the name of Low Frequency (LF) punch and low distortion. The HF loss is made up via the HF record level adjustment.
- American EQ standard is NAB for 15IPS and AES for 30 IPS.
- Euro EQ is called either CCIR (seventies) or IEC
- European EQ hides noise better.
Good luck, and may the Electron Goddess bless your tweaker.