apolivka@x102a.harris-atd.com (polivka al 60047) (07/15/90)
Can anyone answer some or all of the following questions about Dolby C
and dbx Noise Reduction systems?
dbx
---
1. I understand that dbx is basically a compander.
- Is the signal simply passed through a non-linear device such as a log
amp, or is there a dynamic gain device such as an AGC (automatic
gain control), or what?
- If it uses a dynamic gain device, then:
- How does the playback expander know when and by how much to expand?
- Is there a separate control signal recorded that is used to
dynamically adjust playback level?
Dolby C
-------
1. How does Dolby C work?
Comparison of the two
---------------------
1. I've seen advertised SNR's of 85 dB for dbx. What SNR performance
does Dolby C achieve?
2. What are the pros and cons of Dolby C vs. dbx, particularly with
respect to the application of multitrack recording?
3. The Nov '88 issue of Keyboard magazine, p147, mentions a problem
with dbx having a "tendency to cut off leading edges of transient
sounds, such as drum attacks and so forth."
What causes this problem and how serious is it?
Thanks,
Al
--
------------------------------------------------------------------------
Al Polivka arpa: apolivka@x102a.ess.harris.com
Mail Stop 102-4858 usenet: uunet!x102a!apolivka
Harris Corporation phone: 407-729-2983
Government Aerospace Systems Div. Bldg: 102 Room: 3433
P.O. Box 94000
Melbourne, FL 32902
------------------------------------------------------------------------tyager@maxx.UUCP (Tom Yager) (07/16/90)
In article <APOLIVKA.90Jul15102745@x102a.harris-atd.com>, apolivka@x102a.harris-atd.com (polivka al 60047) writes: > Can anyone answer some or all of the following questions about Dolby C > and dbx Noise Reduction systems? > [ ...scientific questions left to someone more knowledgeable... ] > 2. What are the pros and cons of Dolby C vs. dbx, particularly with > respect to the application of multitrack recording? I trust my ears, and I like dbx better. This is a "love or hate" question, and people on both sides tend to have pretty strong opinions. To me, Dolby C seems to trample on highs, and makes quiet passages in the music almost inaudible. For music that remains at a fairly constant volume, its noise-reduction is obvious, and can be impressive. dbx gets out of the way of quiet passages of music, while clamping down on periods of silence. I use it for all the tapes I listen to in my car, and I like the results. My four-tracker (fostex) has Dolby C NR. The music I do is mostly quiet, melancholy stuff that C just tramples to death. I switch it off, and reduce the noise during the mixdown with dbx. It isn't perfect, but I just couldn't get Dolby C to behave itself. I expect that traditional rock/pop would have no trouble punching through the hiss gauntlet that Dolby lays down. On the other hand, as you noted, dbx sometimes doesn't move fast enough. If you were to lay down a quiet percussion track with, say, a closed hi-hat every half-note, you might lose the leading edge of the sound to dbx. It can also be heard to "breathe;" some of the hiss comes through when dbx adjusts itself to quieter sounds. To each their own. If you're writing a paper on the two technologies, good luck in gathering your facts. If you're making a choice between them in preparation for spending a wad of cash on equipment, I'd say you should trust your own ears. Go to a dealer and have them set you up with recorders on both sides of the NR fence. Get some good headphones and go to town. I'd almost guarantee that, after a test drive, you'll have a clear preference for one. > Al Polivka arpa: apolivka@x102a.ess.harris.com -- +--Tom Yager, Technical Editor, BYTE----Reviewer, UNIX World---------------+ | NET: decvax!maxx!tyager -or- uunet!bytepb!maxx!tyager | | I speak only for myself "UNIX: It's not a job, | +-------------------------------------it's a Jihad!" -co-worker------------+
roskill@cs.umass.edu (07/16/90)
In article <56@maxx.UUCP>, tyager@maxx.UUCP (Tom Yager) writes... >I trust my ears, and I like dbx better. This is a "love or hate" question, >and people on both sides tend to have pretty strong opinions. > >To me, Dolby C seems to trample on highs, and makes quiet passages in the >music almost inaudible. For music that remains at a fairly constant volume, >its noise-reduction is obvious, and can be impressive. I think it depends on the quality of each type of the noise reduction. In my opinion, Dolby C on four-track recorders is far superior to the dbx built into similar units. This is not to say I don't like dbx. I love stand-alone dbx units, but the "one-chip" noise reduction dbx suck. My Fostex 160 has Dolby C and I have found it very affective in reducing noise without the "breathing" I find in dbx 4-track units. I guess I'm waiting for Dolby S and a 4-track DAT. Damian |----------------------------------------------------------------| |"Party For Your Right To Fight" | "Welcome to the Terrordome!"| | Damian Roskill | | | Specular International, Inc. | The Makers of Infini-D! | | Roskill@cs.umass.edu | Raytracing for the Mac II | | My opinions are my own..... | "Catch some rays!" | |----------------------------------------------------------------|
fc@lexicon.com (Frank Cunningham) (07/18/90)
> dbx If I recall the term correctly, both dbx and dolby are homomorphic filters, in that their control is derived from the signal itself. They are both invertable - the expansion circuits do the inverse of the compression circuits. They both are reasonably resistant to corruption of the encoded signal by the channel, although Dolby is more sensitive to high-frequency losses since it only works on them. dbx uses a log amp to control the gain of a VCA so the conversion is linear in dB. dbx also bandpasses to control signal side chain so that it is more resistant to frequency responce variations than a full-band control signal would be. > - How does the playback expander know when and by how much to expand? > - Is there a separate control signal recorded that is used to > dynamically adjust playback level? No separate signal. It simple expands linearly in dB the signal it previously compressed. There are some time constants in RMS convertors that are matched. > 1. How does Dolby C work? By some double compression of the high end only, above some frequency, below some threshold (where dbx is level independent for reasonable setups). I've forgotten the details. It's basically Dolby B done twice. > 2. What are the pros and cons of Dolby C vs. dbx, particularly with > respect to the application of multitrack recording? I've never actually compared them. For a reasonable machine and most signals either will do. Dolby C must be calibrated probably to better than 1/2 dB. see dbx#3 below > 3. dbx having a "tendency to cut off leading edges of transient > What causes this problem and how serious is it? I think this occurs when you are pushing the channel. The RMS / log control circuit allows the edge to pass uncompressed to the tape, which saturates. The expansion circuit then cannot restore the original edge. -- -Frank Cunningham smart: fc@lexicon.com phone: (617) 891-6790 dumb: {husc6,linus,harvard,bbn}!spdcc!lexicon!fc snail: Lexicon Inc. 100 Beaver St. Waltham MA 02174 Crime doesn't pay ? Ask thousands of Reagan era bankers.
burley@world.std.com (James C Burley) (07/18/90)
In article <590@lexicon.com> fc@lexicon.com (Frank Cunningham) writes: > 3. dbx having a "tendency to cut off leading edges of transient > What causes this problem and how serious is it? I think this occurs when you are pushing the channel. The RMS / log control circuit allows the edge to pass uncompressed to the tape, which saturates. The expansion circuit then cannot restore the original edge. I'm not an audio techie, but when I used dbx for encoding cassettes of wide-dynamic-range LPs (later CDs) of classical music, I got the impression that the pumping was due primarily to a loss of highs (compared to the rest of the spectrum) on the tapes. No matter at what level I recorded, for example, Telarc's recording of Carmina Burana, the opening number clearly pumped as the cymbals crashed on "Statu variabilis". My theory at the time was that the compression (reduction of volume in this case) kicked in at a level based primarily and initially on the volume of the high-frequency cymbal crash and then primarily and subsequently on the volume of the lower-frequency orchestra and chorus, as the cymbal crash faded out (being struck). On playback, if the highs ended up at a lower level on the tape, the instant when the cymbal crash was the loudest thing would play back at a slightly lower, and normally inaudible, level (without dbx). But two things made the pumping sound audible: dbx multiplied the error by a factor of 2 (since it uses 2:1 compression or thereabouts); and the rest of the sounds, unlike the cymbals, were non-percussive so the change from an artificially lowered volume level immediately back to the normal level (as the cymbals faded and the lower-frequency and more consistenly level orchestra and chorus became the primary volume base) was much more noticeable. In other words, it sounded like the chorus and orchestra did a very quick forte-to-fortissimo step right as the cymbal crash faded from its initial powerful sound (about a quarter of a second, I'd say). Piano music also had a consistent pump on it in the tapes I made for playing in my car. Strongly hit piano notes have a very sharp transient; perhaps they too have, at the transient point, significant high-frequency content, or perhaps they trigger the saturation Frank refers to. Of course, I could be all wrong in my theory -- I've had personal experience of the extremely high quality of Lexicon's audio engineering staff, so I'm not about to suggest that I know more about anything audio than a Lexicon employee! My highest quality stereo is in my car, by the way, so it wasn't that system that was causing the pumping -- it was clearly dbx. Non-dbx recordings of the same piece didn't have the pumping but had the expected large noise floor (which was why I switched to using dbx), and needless to say, once I put a CD player in the car, all those pumping problems went away. Now the cymbal crashes and the chorus sings straight out!
scott@bbxsda.UUCP (Scott Amspoker) (07/18/90)
In article <590@lexicon.com> fc@lexicon.com (Frank Cunningham) writes: >[good info about noise reduction deleted] >> 2. What are the pros and cons of Dolby C vs. dbx, particularly with >> respect to the application of multitrack recording? > >I've never actually compared them. For a reasonable machine and most >signals either will do. Dolby C must be calibrated probably to better >than 1/2 dB. see dbx#3 below Thanks for pointing this out. Although I have nothing against Dolby C, I've found that it is not as forgiving as DBX when dealing with calibration errors. Dolby C tapes made on one deck don't always sound very good played back on another deck. (Of course, this is usually not a problem with high-quality decks). -- Scott Amspoker Basis International, Albuquerque, NM (505) 345-5232 unmvax.cs.unm.edu!bbx!bbxsda!scott
edwardm@hpcuhc.HP.COM (Edward McClanahan) (07/19/90)
Frank Cunningham writes: > If I recall the term correctly, both dbx and dolby are homomorphic > filters, in that their control is derived from the signal itself. > They are both invertable - the expansion circuits do the inverse of > the compression circuits. They both are reasonably resistant to > corruption of the encoded signal by the channel, although Dolby is > more sensitive to high-frequency losses since it only works on them. > ... > > 1. How does Dolby C work? > By some double compression of the high end only, above some frequency, > below some threshold (where dbx is level independent for reasonable > setups). I've forgotten the details. It's basically Dolby B done twice. I haven't been following the discussion Frank is responding to, but I think I can shed some light on the comparison. The basic premise of both Dolby B and Dolby C is to PRE-emphasize the highs prior to recording so that on playback, both the highs from the original signal and the tape hiss can be DE-emphasized. This should have the effect of restoring the highs to their original signal strength (relative to the lows and mids) while lowering the tape noise. The idea of DBX is to "compress" the signal by a factor of 2-to-1 prior to recording. Then, when played back, the signal is "expanded". This strategy has two advantages. First, because the signal is compressed, the recording level (say of the quieter passages) can be higher relative to the tape noise than if the signal was not compressed. Second, the tape noise is more noticeable during the quiet passages anyway, so the DBX expansion circuitry will lower BOTH the original signal (back to its original level) AND the tape noise. Another VERY significant difference between Dolby B/C and DBX (and one where the "improved" Dolby C suffers even more than Dolby B) is in reaching the limits of frequency response. Have you ever noticed that most tape and tape recorder/players quote their S/N at 0 dB and their frequency response at -20 dB? Using DBX allows one to not have to record at such a high level to avoid tape noise thus improving frequency response. Try this experiment at home. You will notice that if you record at -20 dB the highs will be much clearer (not deadened) but the tape hiss will be higher than you may want. DBX avoids this problem. Still, DBX has one MAJOR drawback. While Dolby B/C imposes a fixed boost to the highs (although the amount varies with frequency), DBX varies between emphasis and de-emphasis based on signal strength. Older DBX units were subject to a condition known as "pumping", basically a result of not being able to accurately decide (based on the signal) when to emphasize and when to de-emphasize. This problem, along with higher cost, is why you don't see many DBX units out there. The other reason is that tapes recorded with DBX will not sound at all good when played back without it. Dolby B/C tapes do sound okay when played backk without it (in fact, some people prefer this even though they could achieve nearly an identical effect with their treble control or an equalizer). I hope this helps the discussion... =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Edward McClanahan Hewlett Packard Company Mail Stop 47UP -or- edwardm%hpda@hplabs.hp.com 19447 Pruneridge Avenue Cupertino, CA 95014 Phone: (408)447-5651
bill@vrdxhq.verdix.com (William Spencer) (07/20/90)
in article <APOLIVKA.90Jul15102745@x102a.harris-atd.com>, apolivka@x102a.harris-atd.com (polivka al 60047) says: > Can anyone answer some or all of the following questions about Dolby C > and dbx Noise Reduction systems? > 1. I understand that dbx is basically a compander. > - Is the signal simply passed through a non-linear device such as a log > amp, This would cause distortion, as would anything acting instantaneously on the signal. Therefore the signal is averaged -- NOT an instantaneous process -- to control gain. > - Is there a separate control signal recorded that is used to > dynamically adjust playback level? No, just audio. A seperate signal might be a good idea though. > Dolby C Dolbys are compandings like dbx but to the highs only (different versions) except for the HX pro circuit which plays tricks on the bias. [...] > 3. The Nov '88 issue of Keyboard magazine, p147, mentions a problem > with dbx having a "tendency to cut off leading edges of transient > sounds, such as drum attacks and so forth." > What causes this problem and how serious is it? This is caused by the non instantaneous action descibed above. in article <56@maxx.UUCP>, tyager@maxx.UUCP (Tom Yager) says: > I trust my ears, and I like dbx better. This is a "love or hate" question, > and people on both sides tend to have pretty strong opinions. I trust my ears and I like niether best, usually. With no noise reduction the sound is just so much open and natural. Many peaple record with NR and play back without. This is NOT noise reduction, this is using the Dolby as a compressor or HF "enhancer". This is an example of not liking Dolby but not being willing to admit it. Depends on the music, sometimes you can't get by without NR. > To me, Dolby C seems to trample on highs, and makes quiet passages in the > music almost inaudible. For music that remains at a fairly constant volume, > its noise-reduction is obvious, and can be impressive. But if the level is constant, who needs NR? NOTE: I've noticed that on Portastudios the recording level is lower than conventional stereo recorders (Tone is O.K. therefore azimuth is O.K.). Therefore dbx may be considered a must. Has anyone else found the levels to be different? bill S.
alex@bilver.UUCP (Alex Matulich) (07/23/90)
In article <APOLIVKA.90Jul15102745@x102a.harris-atd.com> apolivka@x102a.harris-atd.com (polivka al 60047) writes: > dbx > --- >1. I understand that dbx is basically a compander. > - Is the signal simply passed through a non-linear device such as a log > amp, or is there a dynamic gain device such as an AGC (automatic > gain control), or what? > - If it uses a dynamic gain device, then: > - How does the playback expander know when and by how much to expand? > - Is there a separate control signal recorded that is used to > dynamically adjust playback level? A compander basically consists of a variable-gain amplifier and a level detector. The playback expander knows when and how much to expand by comparing the signal to a refernece level, usually the -5 dB VU level. The voltage-controlled amplifiers (there are often two in both the compress and expand circuits are also frequency-weighted, along with the level sensors for each circuit. There is no separate control signal recorded that is used to dynamically adjust the playback level. Mine has a calibration control where you record a -5 dB 1 KHz tone on a tape, play it back, tweak a trimmer pot, and repeat until the playback level is -5 dB. -- /// Alex Matulich /// Unicorn Research Corp, 4621 N Landmark Dr, Orlando, FL 32817 \\\/// alex@bilver.UUCP ...uunet!tarpit!bilver!alex \XX/ From BitNet try: IN%"bilver!alex@uunet.uu.net"
ee299bw@sdcc6.ucsd.edu (Unbroken Chain) (07/24/90)
In article <827@bilver.UUCP> alex@bilver.UUCP (Alex Matulich) writes: >A compander basically consists of a variable-gain amplifier and a level >detector. The playback expander knows when and how much to expand by >comparing the signal to a refernece level, usually the -5 dB VU level. > [...] >adjust the playback level. Mine has a calibration control where you >record a -5 dB 1 KHz tone on a tape, play it back, tweak a trimmer pot, >and repeat until the playback level is -5 dB. With dbx, unity gain occurs at 0 dB. As it turns out, the calibration of this is not super critical, as dbx's behavior is linear. That is, the difference between any two signal levels is doubled on expansion. The Dolby schemes are different - Dolby's behavior is dependent upon signal level and varies across the audio spectrum. Therefore, calibration is critical because mistraking Dolby will either not expand and deemphasize high frequencies enough, or it over-expand and overly deemphasize them.... hence the urban legend that Dolby NR merely removes all high frequency content. Dave -- *********************** Dave Chesavage **************************** * dchesavage@ucsd.edu * * "Earth is 99% full. Please delete anyone you can." *
a577@mindlink.UUCP (Curt Sampson) (08/03/90)
> muller@Alliant.COM writes: > > > dbx > > - If it uses a dynamic gain device, then: > > - How does the playback expander know when and by how much to expand? > > - Is there a separate control signal recorded that is used to > > dynamically adjust playback level? > >Don't know. > > This has been answered before, but... In general, expansion on playback uses > the signal itself compared an aboslute reference level. There is > no control signal. I am not sure exactly what dbx per se does. dbx compress on record and expands on playback with a fixed 2:1 ratio. I believe the reference level is -5 dBV for type II (cassette) dbx. > > Dolby C > >My understanding is this... Dolby B is much like an enhancer in that > >it boosts the highs before putting them on tape. Then on playback, B > >cuts them again. My understanding is that Dolby C is much like Dolby > >B applied twice to the same signal. > > No. It *can't* work that way. If that was all there was to it, you > could just establish an appropriate EQ curve and use it. In fact, you > should already be trying to saturate the tape, so just boosting the highs on > recording and reducing them on playback will saturate things more. > There must be some dynamic effect, but there is some question of what > frequencies you are using to measure your level and whether you compress all > the frequencies. This is the basic difference between Dolby B and C, > and possibly dbx too. Maybe someone should post that again? I know it > just went up a few weeks ago, but... AAAAAaaaaarrgh! Dolby B raises the level of all signals above a certain frequency (1500 Hz, I belive) when the level falls below a certain point. Otherwise it does not touch the signal. Dolby C works in a similar manner except that it uses two seperate "zones," from 1.5 KHz to 6 KHz, and above 6 KHz, I believe. The original Dolby A used five "zones" spread across the frequency spectrum. -cjs ( Curt_Sampson@mindlink.UUCP )
rich@sendai.sendai.ann-arbor.mi.us (K. Richard Magill) (08/03/90)
disclaimer: I'm far from an expert, but I've owned and used both a
Yamaha mt2x and a fostex 280.
In article <APOLIVKA.90Jul15102745@x102a.harris-atd.com> apolivka@x102a.harris-atd.com (polivka al 60047) writes:
dbx
---
1. I understand that dbx is basically a compander.
My understanding is that is is a compander that dynamically adjusts to
the signal level attempting to saturate the tape constantly.
- If it uses a dynamic gain device, then:
- How does the playback expander know when and by how much to expand?
- Is there a separate control signal recorded that is used to
dynamically adjust playback level?
Don't know.
Dolby C
-------
1. How does Dolby C work?
My understanding is this... Dolby B is much like an enhancer in that
it boosts the highs before putting them on tape. Then on playback, B
cuts them again. My understanding is that Dolby C is much like Dolby
B applied twice to the same signal.
Comparison of the two
---------------------
1. I've seen advertised SNR's of 85 dB for dbx. What SNR performance
does Dolby C achieve?
I think I've seen dbx rated generally at about 2db above Dolby C in
the equipment I've shopped for.
2. What are the pros and cons of Dolby C vs. dbx, particularly with
respect to the application of multitrack recording?
3. The Nov '88 issue of Keyboard magazine, p147, mentions a problem
with dbx having a "tendency to cut off leading edges of transient
sounds, such as drum attacks and so forth."
What causes this problem and how serious is it?
I notice it, but I notice something worse. between the transients I
hear superflous very annoying hiss. My drum track should sound like
"boom boom boom" but what I hear is something more like "boom
hsssSSSS!S!S!!! boom hssssSSSSS!S!S!!!! boom".
So I've switched to Dolby C.muller@Alliant.COM (Jim Muller) (08/03/90)
In article <RICH.90Aug2135024@sendai.sendai.ann-arbor.mi.us> rich@sendai.ann-arbor.mi.us writes: > dbx > - If it uses a dynamic gain device, then: > - How does the playback expander know when and by how much to expand? > - Is there a separate control signal recorded that is used to > dynamically adjust playback level? >Don't know. This has been answered before, but... In general, expansion on playback uses the signal itself compared an aboslute reference level. There is no control signal. I am not sure exactly what dbx per se does. > Dolby C >My understanding is this... Dolby B is much like an enhancer in that >it boosts the highs before putting them on tape. Then on playback, B >cuts them again. My understanding is that Dolby C is much like Dolby >B applied twice to the same signal. No. It *can't* work that way. If that was all there was to it, you could just establish an appropriate EQ curve and use it. In fact, you should already be trying to saturate the tape, so just boosting the highs on recording and reducing them on playback will saturate things more. There must be some dynamic effect, but there is some question of what frequencies you are using to measure your level and whether you compress all the frequencies. This is the basic difference between Dolby B and C, and possibly dbx too. Maybe someone should post that again? I know it just went up a few weeks ago, but... AAAAAaaaaarrgh! -- - Jim Muller