Monday, June 27, 2011

Thoughts about vinyl records and advanced resolution digital

On a computer group, the subject of vinyl records came up...quite a few digital geeks are quite fond of vinyl records.  I was asked to comment, and came up with all this (never miss a chance to push my favorites either):

I think vinyl is fine.  However, I don't actually bother with it much anymore.  Sacrilegiously most spins of my turntable in the past 4 years have been to digitize recordings not available any other way.  My brother-in-law who is in the business (for 25 years now) of selling audio tweaks like CD dampers (I'm not interested) and is therefore a truly over-the-edge audiophile (yes, he has the ionizers and all that) often praises vinyl but, so far as I know, hasn't had a working turntable in past 5-10 years.  My working vinyl playback system burned out in power surge/brownout last November, I mean to get it fixed (in fact, I have acquired parts for a second system, just haven't gotten around to fiddling with it, top quality vinyl playback typically requires a lot of fiddling).

Of course all analog systems like vinyl are essentially "infinite resolution."  It is unfair to compare them to digital systems on the basis of S/N or similar specs.  Vinyl playback is typically much noisier than digital, if you really work hard you can get 75dB S/N, whereas even standard CD gives you a theoretic 98dB, but it's not really that simple.  The true resolution of a pure analog (vinyl or tape) system (which hardly anyone does anymore, even vinyl record sellers) would be limited by quantum physics alone, somewhere in the neighborhood of 400dB.  (This is echoing tweeks comment that you are hearing a literal echo of the original...yes you are.)  That isn't actually achieved because the "infinite" resolution is obscured by various noise and distortion mechanisms which digital systems can avoid.  However, it turns out we can hear through the analog noise (but not the digital noise).  Exactly how far we can hear through the noise depends on the person and many other factors, but 20dB or so can be demonstrated, and it might be more than that.  So you may well get better resolution from a vinyl listening experience than an uncompressed CD quality one.  And don't get me started about lossy compression systems like MP3, I can't stand any of them.  Lossless digital compression system like MLP are fine, but I've never heard MP3 etc. which didn't sound distorted.

And then there is the care and feeding of a vinyl system which some people enjoy, something like doing an I Ching divination the old fashioned way, with sticks.  That may work for some people, but it does nothing for me, I have enough stuff I need to do with my playback system already.

But possibly more applicable is that generally vinyl records are simply "mastered" differently than digital recordings.  That means they often add equalization and limiting and things like that.  Often, actually, the effect of these things results in a more enjoyable recording.  When digital recordings are made, sometimes the mastering engineers either leave it alone or--more likely--apply extreme amounts of compression to make it as continuously loud as possible, so that the loudness level varies from about 0dB (maximum) to -3dB.  That isn't done on analog systems because the 0dB spot is kind of bad, and distortion increases a lot right there, so they generally keep the level fairly low and just let it hit 0dB for the real super peaks where you don't notice the distortion so much anyway.  The totally annoying level of compression used in many digital recordings (because they can), especially pop ones, is something that audiophiles and the bettering mastering engineers are enraged about.

But digital systems need not be limited to the old 16 bits 44.1 kHz format pioneered by Phillips and Sony.  Nowadays, many digital recordings are originally made at 24 bits resolution and 96kHz if not 192kHz.  Can you hear the difference?  Maybe.  You may not actually need the high frequency response on a 96kHz sampling system (theoretic frequency response to 48kHz) but it seems to help playback if the super-steep brickwall filters at 21kHz are not needed.  And because the distortion in digital systems at very low levels, getting close to the least significant bit, is very nasty, you want to stay as far away from that as possible.  Just the small jump from 16/44.1 to 24/48 gives you most of the audible benefit.

Two audiophile formats appeared on the market around 2000, SACD and DVD-Audio.  Both of these give you something like 48kHz sampling rate or better, and approaching 24 bits of resolution.  Because SACD is a sampled but non-PCM format, it can't strictly be compared with ordinary PCM systems by the usual specs, but many audiophiles believe it is better.  (I have mixed feelings about SACD, long story, but when available, I get the SACD version if for no other reason than it is probably mastered better.)  The "advanced resolution" stereo formats available on DVD-Audio are a clear win, you can't argue that 24/96 isn't simply better than 16/44.1.  And then both of these formats offer a no-lossy-compression multichannel experience in many cases, but like many audiophiles I almost always use stereo which really provides all the spatiality you really need--multichannel for music is mostly distraction.

Unfortunately, the specs for DVD-Audio weren't fully worked out at the time DVD-Video was released, and when they were worked out there were extra licensing fees.  As a result, most DVD players will not play the advanced features in DVD-Audio discs.  And for that and other reasons, DVD-Audio has never hit the big time, and most people, even serious AV people, may never have heard of it.

As for me, I will not buy any disc player that does not support these formats.  For that reason, I did not buy a cheapie Blu Ray player, or any Blu player, until I could get the Oppo BDP-95 (a super machine).

King Crimson is in the process of releasing their early catalog in DVD-Audio.  You can enjoy up to 24/48 on any DVD player, higher levels of resolution require a DVD-Audio player, and I think they even include a bonus CD.  I very highly recommend these "40th anniversary edition" packages, especially the absolutely essential Court of the Crimson King.  Some of their other albums are just a bit hard to get into for most people.  There was absolutely nothing wrong with the 30IPS half track analog tape machines used to record such material originally (though it often suffered from lousy mixing procedures...that was especially true of Court, and the effect has been negated in the new edition with all new mixing).

You can now get digital downloads in 24/96 and 24/192 digital, for playback via computer or you can burn to DVD-Audio disc.  It sounds like a great idea, but I haven't done it yet.

So I stroke my "I want better than CD quality" feeling more by getting advanced digital recordings.  No, they do not have 400dB theoretic resolution, but they seem to have enough resolution, finally.  And generally audiophile editions of all recordings have less compression.  One of my biggest finds last year was Pink Floyd "Meddle" on Mobile Fidelity Gold disc.  It's just standard digital, but the straight-from-the-mastertape uncompressed recording is mind blowing, completely unlike any Floyd you've heard on regular CD's.  BTW, the going rate for these now collectable discs is $100-$300.

But if you find what you need in vinyl, that sounds fine to me.  I've heard lots of mind blowing audio on turntables and big reel tape machines.  One of the hottest sounds I heard at CES in 2009 was from a big reel tape machine (played back by the original recording engineer).

Ferrite vs Iron chokes

According to Tim: Ferrite has less hysteresis than iron, but saturates sooner.

The equivalent to hysteresis in capacitors is dielectric absorption and dissipation factor.  DA causes phase shift, DF causes amplitude reduction.  But levels are much smaller than hysteresis in ferrite and iron chokes.

The inductors in Quad ESL-63 are air core.

Friday, June 24, 2011

Isolating the ferrite inductor

In my previous design for notch-blend filter, I by necessity use a ferrite core inductor, since values like 127mH would be mind bogglingly huge in air core (even a 10mH air core weighs a bunch and sells for $77, now imagine having 13 of those).

Maybe I'm thinking about this wrongly, but it seems it might help the midrange to "isolate" the choke by putting capacitors on either side, instead of just on one side.  So instead of having one 1800pF capacitor, I could have two 3600pF capacitors, one on either side of the choke.  Thus only very high frequencies see the inductor in the first place.

The reason why this might not be true is that electrical circuits are "networks", and one part doesn't see other parts until they are actually in a full circuit, and once there is a full circuit, it doesn't matter where you put the nasty part, the nastyness will be everywhere.

Or another reason might be that even with the capacitor just on one side, the choke is effectively out-of-circuit at high freqencies.

Friday, June 10, 2011

Pioneer F-26 FM tuner front end

One of the best.  Here it is:

Designing the notch-blend filter for KPAC

So far, no response from KPAC engineer, and station hasn't changed.  So I'm developing a necessarily imperfect solution on my end.  Many high end tuners have a high blend switch, very useful in cases like this.  The Pioneer F-26 doesn't.  But the best possible solution on my end might not be a high blend filter but a "notch-blend" filter that specifically notches the 10.5 kHz buzz into mono where it vanishes.

Here is a great online RLC notch filter calculator.

The difference between notch and notch blend is that there are two series resistances, one for the left and right outputs (which I believe in a simplistic model are simply seen in series and summed), and the LC network bridges them.  At least that's my guess now.

Set for notch at 10500 Hz, and Q of 2 which seems like a good compromise between specificity of filter and ringing.

One possibility is 1.6k total resistance, then you get 0.0047uF capacitor and 0.488H inductor.  Based on this calculation, I found and ordered a Russian Teflon 0.0047uF capacitor on ebay for $2 (plus $7.30 shipping).  Was all set to order 47 mH sheilded RF inductor from Mouser when I decided to check F-26 schematic and specs.

Turns out F-26 has output impedance spec of 2.2k ohms and that looks to be correct.  Each output from the final AF amplifier chip sees a voltage divider with two 4.8k resistors in series.  The output from that is further shunted by 10k ohm pot for the variable outs.  Given some unknown driving impedance of the AF amplifier chip, this could quite reasonably add up to 2.2k ohm impedance, so I'll go with the spec.

It looks like Pioneer is doing several things with this output design.  They are cutting the output voltage in half (and thereby also cutting amplifier noise in half).  They are making it bullet proof so any fool can simply sum the two outputs to mono without increasing distortion.  And they are adding some HF rolloff (aka decoupling) which is determined by the capacitance of the cable connected (making it somewhat tunable).  My calculations show a 240 pF cable (cheap 6' cable) would produce a cutoff frequency of 300kHz, most likely not important.  And a good short cable could have 24pF, giving a cutoff of 3Mhz.  However, a 1200pF cable (and they are out there as audiophile magic cables) would produce a cutoff of 60kHz, which sounds OK until you consider that you must have a cutoff frequency of 10x higher than passband to have less than 0.1dB attenuation anywhere in that passband.  So a 60kHz cutoff starts to get close to 0.1dB attenuation at 6kHz.  That could well be noticeable.  The 10x effect was something I discovered the hard way, after decades of using relatively high resistance passive level controls instead of active preamps.  For some reason, my output pots always sounded a bit dull, but I couldn't believe it, because my calculations said I had cutoffs in the 50-150kHz range)  200kHz bandwidth sounds like over-the-top audiophile madness but isn't.  And then there is also the way successive rolloffs in an audio system add up...

Normally I don't like that sort of output design the Pioneer uses, I like true low impedance outputs, but for an FM tuner, producing lots of HF noise anyway, some of which may be supersonic, 2.2k ohm output impedance is quite possibly desireable.  A feature.  Anyway, the claim has always been that Pioneer has the most transparent midrange.  Indeed the AF amplifier is protected from output loading distortion, which might help that.  The claim has generally not been that the Pioneer has the most transparent highs.  Kenwood tuners often achieve that distinction (and many don't like the resulting sound, bright by comparison with other tuners).  Kenwood was so obsessive about low impedance outputs that my 600T tuner has a separate audio amplifier chip after the variable output pot.

End of digression.  I must therefore design my filter around two 4.4k ohms (the channels in series).

Looks like this combination works:

0.0017uF (1700pf, calculated)
0.135H (133mH, nearest E24 value)
Resistance 4.45k (calculated R)
Q 2
Fc 10500

Closest standard capacitor value is 1800pF, just ordered some Russian teflons.

That changes somethings, actually R at 4.4kohms is probably not exact anyway probably slightly lower, and Q is not critical.  Worst effect is that Fc is lowered to 10210, and that's pretty much determined by L and C.

R=4.42 ohms

So with 1800pf, looks like I I need 0.127H inductor instead of 135 for best result.

Wednesday, June 1, 2011

Letter to KPAC

Dear KPAC Broadcast Engineer,

I have been bothered by a high pitched buzz when playing KPAC in stereo, but not in mono.  After doing a number of experiments, I have determined that there is a spurious 10.5 kHz tone in the L-R stereo multiplex subcarrier as broadcast by KPAC.  I have digitally recorded a silent interval, and found the problem can be eliminated with a 10.5kHz digital notch filter having a Q of 10.  A slight peak there is also visible in a 12th octave spectrum analysis of L-R.  Though it is hard to see in a spectrum plot amidst the random noise, it is annoyingly audible.

The problem does not seem to originate in any of my tuners, my stereo, or my receiving location.  I have tried several different tuners, in two different locations including one fairly close to the transmitter, and have also tried using only headphones.  I do not get this problem on any other station, including stations with HD or RDS subcarriers.

The tuners I have used include some of the best ever designed, including a Kenwood KT-6040 that was designed in the 1990's to handle the RDS subcarrier at 57kHz; it is described as "birdie free" by well known FM experts.  I have also used a Pioneer F-26, highly praised by FM experts as one of the 3 best tuners ever made, and which was fully refurbished and aligned in April 2011 by the nation's leading tuner restorer.  Interestingly, it seems the better the tuner is the more clearly and annoyingly I hear the 10.5kHz buzz.  On lesser tuners, the buzz sounds like a strange kind of hiss which is more easily ignored.  (Or perhaps the strange undulating hiss on those tuners is yet another problem I haven't figured out yet.)

The buzz can be eliminated by listening in mono or using "high blend" to reduce high frequency stereo noise at the expense of stereo separation.  But my favorite tuner, the Pioneer F-26, does not have a high blend control, and I do not like reducing stereo separation.

Many people might not notice this problem because many tuners nowadays auto-blend until all noises are gone; that is what nearly all car radios do now.

By and large, the sound quality and musical content of KPAC is excellent and that is why I have spent so much on analog FM tuners and in investigating this problem.  I hope the problem can be fixed by servicing or refurbing existing equipment rather than buying new digital equipment which may be of inherently inferior quality.  I am especially opposed to the use of HD digital radio, which in my experience is always inferior to old fashioned analog FM radio.

You can read about some of my experiments on my audio blog.  I have also posted questions to the Yahoo group FMtuners, and the verdict was that I should report this problem ASAP to the station engineer.  I am sorry it has taken this long.

I appreciate your attention to this issue, and it would be great if you could keep me informed as to the status  and/or possibility of quick resolution.  I may be able to get help from a network of FM experts if needed (such as obtaining service manuals and insider tips for specific older station equipment).  If you find my analysis to be faulty, please let me know, and in that case I would like to know what subcarriers you use on KPAC, which may be helpful in correcting the problem on my end.  I expect I will shortly be able to post an audio/video demonstration of the problem I have found so far to my blog.  I could email you the 1.7 second L-R wave file now if that would be helpful.

Charles Peterson
TPR Member #