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Messages - stellavox

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46
Tape Tech / Dolby B decoder "troubles"
« on: November 19, 2013, 08:45:18 AM »
A year or so ago, a friend gave me a little box he found at a Boston-area electronics swap meet.  It was labeled "Dolby B decoder tester"  and with it was the  article (reprinted below) from a long-ago issue of the Boston Audio Society monthly journal.

In essence, the article describes a potential sonic problem with most consumer Dolby B decoders.

Have to get technical here to try and explain what goes on.   These decoders operate by sampling the high frequency (HF) content of the incoming audio "stream" and automatically "rolling off" the HF of the incoming audio when there is no "information" there.  In this way the audible effect of tape hiss is decreased.  The decoder senses this information by changing the incoming AC audio information to a DC control voltage that varies the gain of a amplifier in the later audio path to increase or decrease the HF.  Problem is how this DC voltage is detected.  To keep the costs down for consumer equipment, most B decoders used "half-wave" detectors which only sense the "positive" portion of a music signal.  Because music is naturally asymmetric, controlling the HF response of the entire signal based on what's happening on "only" the upper half of its waveshape will change things.

Bottom line is that there CAN BE AN AUDIBLE DIFFERENCE to the high frequencies of music coming OUT of a consumer (not professional) Dolby B decoder depending on the absolute phase of the music going IN to the decoder.  Unfortunately, assessing the audible effect is not easy - you have to devise a way to reverse the phase of each channel's signal before the decoder - then do an "A/B".

Charles   


To "B" or Not To "B"

Inverting polarity can create problems beyond those correctable by simply inverting the signal an additional time.  Inversion can cause gross level errors in signal processors using half-wave detectors, integrators, or any other control circuits that behave asymmetrically.  For a sine wave, the error is predictable, for the energy is symmetrical about the xero-axis.  It may be inherently "corrected" if the circuit was designed-using sine (or symmetrical square) waves as test and calibration signals.  Yet most music, in fact, most sounds are asymmetric.

Dolby-B is a typical problem system.  A simple, but quite representative, music signal shape is a ramp waveform of 2 Khz with a rise time of 100 microseconds and a fall time of 400 microseconds.  There is a 6 dB peak-to-peak level difference at -30 dB relative to Dolby level when such a signal is inverted in the record mode.   It would not be a serious problem if a reciprocal change incurred in the play mode, but a couple of variables conspire to prevent this from happening much of the time: the tape recorder may invert the signal, and there is no convention of correct polarity within the different circuit elements by different manufacturers of Dolby-B units.

In fact manufacturers may not even be self-consistent. The Advocate 101, the Advent 100 and the Advent 100A all differ from each other. In the record mode, the Advent 100 and the 101, if fed a positive-going signal, provide a positive-going signal to the integrator and a positive-going output to the tape recorder.  The Advent 100A, on the other hand, provides a negative-going signal to the integrator and an inverted output to the recorder.  In playback, the 101 again provides a positive-going signal to the integrator and a non-inverted output, provided the tape recorder is non-inverting.   The 100 however inverts the integrator's signal, yet gives a non-inverted output. The 100A does not invert the signal to the integrator, but does invert the output.   But the 100A has already inverted its record signal, so again assuming a non-inverting tape recorder, the integrator really is seeing an inverted signal, and the unit is giving a non-inverted output; it is decoding correctly.

This is a confusing situation, and it is easy to see why manufacturers of Dolby-B's are confused too.  Unfortunately, your tape's sound will become even more confused when a friend plays it back through a Dolby of a different color.  Perhaps this is why Advent 100's sometimes remove high-frequency detail along with tape hiss and why Dolby-B encoded tapes do not always travel well. The differences (with a total of seven possible combinations) occur not only between products once made by Advent, but randomly between those of other purveyors of Dolby B systems as well. The Tandberg Dolby-B, for example ...

Then there is the problem of pre-recorded tapes. The cassette medium is perhaps insufficiently imprecise that one need not be concerned with minor frequency response and envelope tracking errors (if 6 dB is minor?). But how are Barclay-Croker tapes encoded? Fortunately for the recording industry, neither Dolby-A nor the various DBX systems have these problems. 

Another consideration might be that before proposing such a problematic system as Dolby-B for FM broadcasting, standards should be adopted not only for all elements of Dolby-B circuitry but also for all parts of the FM signal chain. The standards would have to include the exciter, transmitter and the FM tuner, its multiplex system, and of course its Dolby decoder.  It might be far simpler to adopt a 50 microsecond emphasis/de-emphasis standard and forget Dolby-B entirely.  As Boston-area listeners know, state-of-the-art FM broadcasting by at least two local stations hardly requires Dolby-B encoding.

Scott Kent (Massachusetts)

47
Events / Re: RMAF 2013
« on: October 03, 2013, 08:00:13 AM »
I'm going to be spinning a few tapes in Dave Cope's Audio Note room on Friday and Saturday evenings just before (and after) the show closes - from 5:30 to 6:30, I believe.  Love to have you'all stop by and chat (or just listen).

Charles


48
Tape Tech / Dolby A update
« on: July 05, 2013, 11:28:34 AM »
I just finished doing a "bunch" of work to some Dolby A decoders; my own DIY decoder (using Dolby Cat 22 cards) and modifications to a pair of 361 units; and wanted to share some thoughts about the process, while I remember them.
 
First, regarding the ?modifications? (which also apply to the original 301 encoder/decoder); I don?t use balanced wiring on my system, so I rewire them for unbalanced operation by mounting input and output RCA jacks on the back panel, and ?bypassing? the input and output transformers by ?lifting? the ungrounded (?hot?) connections to the transformer primaries and connecting them instead to the RCA jacks.  With extra work, you could mount and use one DPDT switch per channel to select either the RCA unbalanced or transformer balanced outputs.

The other modification involves adding a front-panel mounted INPUT LEVEL control to enable you to quickly set the Dolby level when playing back a tape.  Stock Dolby units were mostly used in studio applications, so the input Dolby level control could be ?set? to the studio?s standard level, and ?forgotten?; these controls were typically mounted where they were not readily accessible.

From a ?home playback? perspective, since you are dealing with differing playback electronics with non-standard output levels and potentially varying tape levels, adding some convenient method to set Dolby levels becomes a necessity, IMO.  The 361 uses a 10-turn, 20K ohm, screwdriver adjust control (RV1) to set the Dolby level; the 301 uses a single turn 10K pot (RV101).  RV1 is accessible from the front of the 361; in the 301, RV101 is mounted on the PC board of the plug-in Amplifier module and is not accessible.

The 361, has a narrow, removable plate that covers the opening for the Cat 22 card and two relays to its left.  This plate has two thumbscrews that secure it to the front panel (all the 361?s that I have worked on are missing this plate).  To mount a level control on the front panel, I remove the panel (5 screws ? 3 difficult to get to) and ?wrench out? the left hand cover plate nut attached to the front panel.  This creates a hole in the panel that is ?just? big enough to mount a small 25K single turn (linear) pot.  I then remove RV1 and wire the new pot in it's place on the PC board ?conveniently? mounted maybe 3 inches above and to the right of where the new input level control has been mounted.  On the 301, I drill and mount new 10 K (linear) pots near the (normally burnt out) power ?on? indicator, on the front panel of the Power Supply Module.  You can now hook up the decoder to the rest of your system, play the Dolby tone on a tape and set the L/R levels to the markings on their respective meters.

This of course begs the question if the modules are working properly.  The 301 used a special ?Alignment Extender? test module: you unplug a card from the decoder, plugged this extender module into the spot occupied by the card and re-plugged the card into it. The module contains a selector switch that you rotate to different positions while monitoring test posts on the card with a voltmeter, and adjusting certain board-mounted potentiometers to specified tolerances.   I know the existence of ONE extender module and it?s NOT mine.  The Cat 22 modules have NO user adjustments - all were initially set at the factory using fixed components.  Dolby made a Cat 35 Test Set that a Cat 22 Module could plug into and be checked for proper operation. Good luck finding one of those.
Since I had a total of 8 Cat 22 modules to ?check out?, I came up with a test procedure that seemed to work OK.  My (analog) Amber 4400 test set can output a 20hz to 20Khz swept sine wave (with frequency markers) to a ?device under test? (DUT), and ?drive? a separate oscilloscope to display the swept signal returning from the DUT with up to 0.2dB amplitude resolution.  So I first performed frequency sweeps through the decoder at ?Dolby? level, 10dB and 20dB above it; and 10dB and 20dB below the level just to see what it would look like.  As expected, the sweep flattened out as the level got higher, and displayed maximum boost/cuts at lower levels - indicating that the processing was "working".  For example, at Dolby level, the sweep was flat to within 1.5dB or so, with a 1dB peak around 1Khz (got even flatter at +10.  At -20dB, there was 5dB ?hump? around 150hz, an additional 2dB hump around 2Khz and the upper frequencies rolled off quickly above 8Khz, being around 4dB down at 16Khz.   Now please understand that these measurements are steady state.  There is a LOT more going on in the decoder with respect to processing transient attacks and decays.

My "go/no-go? test on the modules consisted of running them thru both the Dolby level and -20dB sweeps and see if the displays ?looked the same? between two units? if it did, the second unit ?passed?.  4 of the modules ?passed?, two had no output whatsoever. One had the right "curvature" but a high DC offset, and one didn?t seem to process the -20dB level sweep with the total amount of curvature "variation" as the ?good modules.  As I had enough good modules, no repairs were attempted so I can comment on particular problems, but would look for bad coupling caps.  Listening tests seem to indicate proper processing for the "good modules".  I did tweak the good modules by replacing what I consider to be the ?most critical? electrolytics in the signal path, (C533/C541) (with Elna Cerafines).

More later on my adapting these A units to Dolby B

God it's hot here!

Charles

49
General Discussion / 30ips EQ
« on: January 17, 2013, 07:00:43 AM »
I'm doing some "research" on 30ips and am ordering a Calibration Tape from MRL (with other speed sweeps to save money).  Looking over their excellent material, it appears that the time constants for 30ips AES EQ (also called IEC2) are HALF those for 15ips IEC (more accurately IEC1; the EQ that the Tape Project uses).  The info also indicates that if you have a frequency sweep recorded at one of those two speeds/EQ's, it will play "correctly" at the other speed/EQ, and visa versa (of course the reproduced frequencies are all shifted "up" or "down" by a factor of two depending on which way you go).  Important thing here for me here is that the playback equalization circuitry and components can be IDENTICAL. Have I got this right?

THANKS

Charles     

50
Wow Xavier -  NEVER thought about that - too stuck in the past perhaps.   If I ever do another update,  I will include the "new" offerings.

Charles

51
IN the files section of the Yahoogroups Reel_Tapes Blog

Charles

52
General Discussion / Re: IS <-S T E R E O-> DEAD ?
« on: January 14, 2013, 06:36:09 AM »
Maybe Steve;

A few years back when I was finishing up repairing/aligning some FM tuner, I ran it across our local FM band and realized that on one station the FM stereo light didn't come on.  Thought at first that it might be the tuner but it worked OK on other stations.  Happened to run into the station manager some months later and asked him what was happened.  He said that their multiplex generator broke and it took them quite a while to get it repaired, during which time they broadcast in mono.  NO ONE ever called in to complain/ask (besides me, after the fact) - so they didn't rush the repair. 

Guess you don't need stereo in an elevator.

Charles

53
Tape Tech / Re: otari 5050: impedance and output of the playback heads?
« on: December 30, 2012, 11:13:15 AM »
Welcome monotube,

I'd guess that the impedance of the PB head is in the range of 100mH which will work fine into a solid state preamps.  Typical values for tubed PB heads were in the 400mh range.  The higher impedance means that you get a few more dB out of the head which reduces the preamplifier gain requirements.  You could just try hooking the head up to your phono preamp and (recognizing that the EQ network is "wrong") just see if there appears to be enough gain out of it.   it  If so then yes, change the RIAA network to tape (I'd suggest lifting one end and putting in a switch so you can switch back to RIAA if you'd like).  For what it's worth, tape pre's seem to need about 50dB of gain (depending on the "sensitivity" of the rest of your system)  - push a tubed unit more than that and you will start getting into noise issues - perhaps requiring a step-up transformer somewhere in the circuit.  In any case - please experiment - and share your results!

 Good Luck

Charles

54
Prerecorded Tapes / Re: Christmas tape play
« on: December 26, 2012, 07:35:42 AM »
Yes, Thanks Neal

The Nutcracker has always been my favorite Christmas music.  Especially loved the two-tape Westminster, Rodzinski version, but have sadly lost track of it over the years.   In heavy rotation the last week has been (horrors) a CD of carols by a string quartet - John Marks "Rejoice!"

Happy New Year everyone.

Charles

55
We're on for Friday, 12:30!!!

See attached: http://audiofest.net/2012/seminars.php

Love to see any of you who could make it (INTRODUCTIONS OF COURSE) AND would ask any of you who might like to be "part of the fun" to get there say by 12:15.  Paul S. can you make it?

AND OF COURSE we can also discuss drinks / dinner / listening later!!!!
 
Cheers

Charles

56
over on "What's Best":

http://www.whatsbestforum.com/showthread.php?7571-Anyone-up-for-a-reel-to-reel-quot-tape-forum-quot-at-RMAF

Any suggestions / contributions?

Doc /Paul - care to discuss hardware / TP firmware for a few minutes?

Charles

57
Valvmusicus,

In answer to your questions:

Q.  What should Freq Resp be of the effected signal be?   A. nice and flat
Q.  What should the bias levels be  A. enough to get the best signal to noise ratio

Charles

58
Events / read and scroll down a bit
« on: April 21, 2012, 03:27:12 PM »
http://www.stereophile.com/frontpage?page=8

"sing - sing - sing" was a Benny Goodman dub courtesy of Rich Brown

Charles

59
Events / Tape at the NY Audio and AV show - this weekend (Apr 13-15)
« on: April 11, 2012, 05:27:31 AM »
Hi All,

Will be spinning tapes in the Robyatt Audio Room (1524) - Waldorf Hotel, 301 Park Ave.

Including two TP sampler tapes graciously supplied by Paul S.  Please stop by.

Charles

60
Tape Tech / Re: Possible replacement for 5532 opamp?
« on: March 15, 2012, 08:14:30 AM »
Thanks Bill,

Good luck with your "play time", and be sure to look at the output with a scope.

To not tie up this thread with what others might consider minutiae, feel free to contact me - [email protected]

Charles

Forgot - a friend had problems with the tape pre circuit in the National app note

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