DBX Demo Disk


Noise Reduction - DBX

Tuesday, May 28, 2013


While scrounging around for other stuff, I came across an old demo record from dbx. This disk was used to show off the then new dbx professional noise reduction system. The type 1 pro system was aimed at recording studios. The later type II system was for home use.

dbx noise reduction works by compressing the source material's dynamic range by a factor of two before recording on a relatively noisy medium like magnetic tape. On playback, the compressed material contaminated with noise is passed through an expander which restores the original dynamic range. The contaminating signal (tape hiss and record scratch) is attenuated by the expansion process, since the noise is most audible in quiet passages where the expander reduces gain. Because dbx Type I and Type II are single-band companders (unlike Dolby-A's four bands), they are more susceptible to noise modulation and mistracking from differences in the input and reproduced signal. To deal with this, both Type I and II use very strong high-frequency pre-emphasis of the audio signal in both the recording path and the control signal side chain. This causes the compander to 'back off' the gain in certain circumstances and reduce the audibility of noise modulation – even with this pre-emphasis, noise modulation is audible with noisy media such as Compact Cassettes and can be heard in some cases with pro recorders. In the control signal path, the dbx Type II process rolls off both high and low frequency response to desensitize the system to frequency response errors – since the roll-off is only in the control side chain, it does not affect the audio. The dbx Type-II "disc" setting adds an additional 3 dB of low-frequency roll-off in both the audio path and control path. This protects the system from audible mistracking due to record warps and low-frequency rumble, which is clearly a problem in this Type I record.

The dbx Type I system is meant to be used with recording media that have a S/N, before noise reduction, of at least 60 dB and a -3 dB frequency response of at least 30 Hz to 15 kHz. dbx Type-II is for more noisy media that have a lower S/N and restricted frequency response. Both systems use 2:1 companding and provide exactly the same amount of NR and dynamic range improvement – in other words, they provide the same end results, but are not compatible with each other.

So this disk became a favorite and loud item at AES demo booths and some audio shows. It demonstrates both the strength and weakness of noise reduction systems. The “dynamic range” is astonishing for a vinyl record. There is almost no surface noise, but a close listen shows the slow decay of the gong in particular is strongly modulated by record warp and other sounds.

As Rick Crampton puts it,  . . . . and once the initiate gets over the jaw-dropping " dynamic range " of DBX, the warts begin to emerge into focus. I think that DBX was a moderately successful studio tool where it was likely that a single mic or a single instrument was recorded to an isolated track in a virtual anechoic studio environment ( a popular studio design in the 70's ). Any signal with a natural sustain or acoustic decay would ultimately betray the device's confusion at the lower end of dynamic range. Of course a certain amount of this grunge could be swamped by the addition of artificial reverb, particularly if the final mix was NOT DBX encoded. DBX was simply NOT the right tool for recording in a Gothic cathedral . . . Surely you remember the bench test which revealed the " Halo of Crud "??

“NAILS” composed by James Rago


The original dbx Type I and Type II systems were based on so-called "linear decibel companding" - compressing the signal on recording and expanding it on playback. It was invented by David E. Blackmer of dbx, Inc. in 1971.