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dbx Expnders - 3bx, 5bx

This is a new thread which continues from a somewhat unrelated thread that I was pulling off topic.

Hi Sean - re: DBX expanders, three points:

1. The maximum Harmonic and Intermodulation distortion specs for the 5bx are .15%. Which is not small, but it's inaudible compared to the fact that the average vinyl record has had about 30db of compression in the recording and mastering. 30db!!! Now THAT is distortion. If I can remediate that, at a very small sonic cost - for myself, I prefer to. I have tried to hear a substantive enough "negative" difference in a PROPERLY adjusted 3bx or 5bx to know that it's not imaginry, and I can't. But maybe that's me.

2. Expansion/Compression is really a very simple process which in and of itself produces very little in the way of "artifacts". Re-expanding a compressed signal is not a big deal. The only parameters are Transition Level (the db level at which soft is made softer, and loud is made louder etc. and Ratio (the % change to boost or lower volume as a function of deviation from the transition level). As you stated - it's better if you can match the expansion parameters to exactly reverse the compression - but if you can't it's not that big a deal. All you're changing is the relative volumes (amplitudes) of possible related harmonics. You're not introducing phase or time distortion. So you may not be hearing exactly what was recorded, but you're a lot closer than you were listening to the vinyl straight.

In addition - and this is no small advantage - the expansion process by it's nature REDUCES any vinyl noise (which I also consider SERIOUS distortion) very significantly, because it sees it as in the "soft" zone.

3. There is a common misperception that the 1bx expands the entire range, the 3bx expands over 3 freq. ranges and the 5bx expands over 5 bands. This is not true - they all expand the entire spectrum as a whole. The only criteria for expansion are the db levels of the material above or below the transition level. Since the "Transition" level and expansion ratio are user defined, you can get a pretty darn good result. The 5bx makes this very easy, since it has a remote and 5 memory presets. If in doubt - underexpand.

4. They do split the freq. spectrum into bands for the purpose of "Impact Restoration", which seeks to undo the inherently very slow Transient Response of the vinyl media itself and the damage that lazy recording engineers did with Peak Limiters. Now this comes under what you mention as a personal preference - there HAS to inherently be some phase distortion going on here (but again I haven't been able to hear it distinctly.) However they designed these circuits - they did a darn good job. That's the cost. The benefit is the restoration of what a stick hitting a drum actually sounds like. Pop! I'd rather hear that than a phase correct Phoof....

But again - as you said - these are my personal preferences. It's impossible to listen to a vinyl record and hear the "truth". So it is just a tradeoff that I prefer to make. Fix a large amount (about 30%) of one type of distortion while introducing a small amount (maybe 2 or 3%) of another type.
opalchip

Showing 8 responses by eldartford

eldartford
4,262 posts
 
Opalchip....DBX certainly perfected the use of compression/expansion to overcome analog tape recorder noise. They lost out to Dolby labs because of stupid marketing decisions having nothing to do with technical merit.

I do not think that one-way expansion of dynamic range is effective for noise reduction, although you may like it for other reasons. The Phase Linear Autocorrelator (one of Bob Carver's better ideas) is much more effective. (It is a very clever dynamic multiband filter).

The DBX application that I found to be very effective was the DBX LP system, where the recording was very much compressed and use of the complementary DBX expander was essential for playback. Noise reduction was only one benefit. Phono pickup performance was also improved by avoiding any highly modulated grooves. Based on response to prior postings, I must be the only guy who ever experienced this system.
eldartford
4,262 posts
 
Onhwy61...Did you ever hear DBX encoded records? If not, research them.
eldartford
4,262 posts
 
Twl...Too bad. Do you realize that most of the analog tape masters, which audiophiles extol, used DBX noise reduction. It was better than Dolby, which the rest of the master tapes used.
eldartford
4,262 posts
 
Sean...The main problem with dynamic processors of all kinds, noise filters, expanders, multichannel logic decoders, is the fact that their action is controlled by the audio signal, so there is always some delay before they act. The usualy listener complaint is "pumping". Digital technology could eliminate this problem, by delaying the signal for a few millisconds so as to give the processor time to figure out what to do. Then, the processor action could be introduced at exactly the right moment.

The phase 1000 Autocorrelator did several things.
1. Dynamic rumble filter (switched out when audio signal had LF content).
2. Dynamic Multiband HF noise (scratch) filter.
3. Dynamic range expander with very slow action to match human gain riding. It was not designed to undo electronic compression.
4. Peak unlimiter.

It was specifically aimed at LP problems, and it was good for everything except clicks and pops (for which another outboard processor could be used). My complaint was that it did need to be tweeked up for each LP, and that was a nusiance.
eldartford
4,262 posts
 
inpepinnovations@aol.com...The Autocorrelator control signal is LEFT + RIGHT, (Mono). The dominant FM radio noise is in the LEFT - RIGHT multiplexed signal which is mixed with the Mono so as to derive LEFT and RIGHT. Therefore the performance of the Autocorrelator is not good with FM radio. If you can figure out how to do it you might invert one channel going into the autocorrelator, so that the control signal becomes L-R, and therefore a good indicator of noise.

I once had two Autocorrelators, for the front and rear channels of a matrix quadraphonic setup. Of course with LPs it is the L-R signal (vertical groove modulation) that has the big noise problem, and I modified one of my units in this way for the rears.
eldartford
4,262 posts
 
Sean...No disagreement that existing dynamic processors (which I know of) always are slightly behind the signal. However, some are much better than others in this regard. Bob Carver's toy was pretty good, as inpepinnovations@aol.com reports. My point is that digital technology affords the oportunity to overcome this problem, but then digital recordings don't (from a technical standpoint) need the kind of compression that calls for expansion on playback.
eldartford
4,262 posts
 
Sean...That's what pop music fans like. The customer is always right. Real audiophiles listen to classical.
eldartford
4,262 posts
 
Somsel... DBX LPs had the misfortune to arrive at the same time as CDs. Their improvement over regular LPs is large. Focus is mostly on noise reduction, where that compares with CDs, but there are other advantages. Most important IMHO is that performance of the phono cartridge is greatly improved because groove modulation is always in the "sweet spot" for the cartridge. No hard to track passages or imposible to track peaks. Also,because of closer groove spacing playing time is increased.

I have a few of these LPs, but you won't get them! I keep them for demos. Unfortunately there were few recordings issued, and the performances were not the best.
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