Line Up Specification for BBC Reel-to-Reel Tape Decks
BBC Line-up Specification
The British Broadcasting Corporation, or the BBC as it is lovingly known, used to use tape recorders extensively to produce and play out radio programmes. It is only until the early 21st century that production moved away from tape and onto digital playout systems.
This page details the specifications that were originally published in 1984 by the BBC World Service for full-track mono 1/4 inch tape machines using type 200 tape. The specifications were still valid when the tape machines were decommissioned in recent years.
Units and Abbreviations
Throughout this article, the following abbreviations are used:
dB - decibels (these are logarithmic units that express the ratio between two values)
nWb/m - nano Webers per metre (units of magnetic flux density on tape)
ips - inches per second (speed that the tape moves past the tape head)
dB4 - noise measurement peaking to PPM 4 (The amount of amplification required to make the noise read PPM 4)
PPM - Peak Programme Meter (A meter that is used in professional audio to show the level of an audio signal)
rms - root mean square (a statistical measure that gives the value of an equivalent direct current)
Hz - Hertz (cycles per second of a wave)
kHz - kilo Hertz (thousand cycles per second of a wave)
The frequency response is a measure of the output spectrum of a system for a given stimulus. Shown graphically, it would have the frequency on the x-axis and the magnitude on the y-axis, and it would depict how the magnitude varies as frequency increases from a low value to a high value.
With clean heads, the frequency response of the tape machine should be within the following figures:
+/- 1.5 dB (60 Hz to 10 kHz at 7 1/2 ips)
+/- 1.5 dB (60 Hz to 15 kHz at 15 ips)
+/- 2 dB (60 Hz to 10 kHz at 7 1/2 ips)
+/- 2 dB (60 Hz to 15 kHz at 15 ips)
Noise is defined as the unwanted signal in a system. Ideally, we would want this as low as possible.
Noise performance specifications for a tape machine are as follows:
Better than -50 dB4 for machines fitted with full track replay heads
Better than -48 dB4 for machines fitted with 0.180" replay heads
Better than -44 dB4 for machine fitted with full track replay heads
Better than -42 dB4 for machines fitted with 0.180" replay heads
Note that the figures for noise quoted above are with respect to a nominal zero level of 250 nWb/m, 8 dB below the peak flux level of 640 nWb/m.
Wow and Flutter Performance
Wow and flutter, is a measure of the amount of frequency wobble on a system, which is caused by speed variations on the tape transport. This could be caused by worn bearings or motor drive issues or problems with the tape path.
The total rms unweighted Wow and Flutter must not exceed 0.1% throughout the length of any tape spool size.
In order to faithfully reproduce the original recordings, the tape must move at the correct speed.
The BBC specification requires that the tape must be within 0.2% of nominal, throughout the length of any spool size.
Zonal Tape - 1/4" Quarter inch
Total harmonic distortion is defined as the amount of harmonics present from a sinusoidal signal. It is usually measured at 100Hz and 1kHz and then measuring the residual band-limited signal with a filter applied at the original frequency of 100Hz or 1kHz. It can be specified in dB or as a percentage. The lower the distortion, the more accurate the sound.
Total Harmonic Distortion must not exceed 2% overall (34 dB separation) at peak recording level of 640 nWb/m.
spectrum analyser - distortion
As tape is often reused, the erase head must remove the previous signal on the tape before recording a new one. A partially erased signal will affect the newly recorded signal and affect the sound, so it is important that erase performance is good.
The erase field should be sufficient to result in complete erasure of 1kHz tone, recorded at the peak flux level of 640 nWb/m. This should be determined by recording peak level 1 kHz tone and then erasing it, with the record input to the machine terminated. The original 1 kHz tone should not be audible above the noise level.