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
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------------------------------------------------------------------------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!" | |----------------------------------------------------------------|
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." *
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