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Since its inception ten years ago, the Harbeth User Group's ambition has been to create a lasting knowledge archive. Knowledge is based on facts and observations. Knowledge is timeless. Knowledge is human independent and replicatable. However, we live in new world where thanks to social media, 'facts' have become flexible and personal. HUG operates in that real world.

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Questions and Posts about, for example, 'does amplifier A sounds better than amplifier B' or 'which speaker stands or cables are best' are suitable for the Subjective Soundings area.

The Moderators' decision is final in all matters regarding what appears here. That said, very few Posts are rejected. HUG Moderation individually spell and layout checks Posts for clarity but due to the workload, Posts in the Subjective Soundings area, from Oct. 2016 will not be. We regret that but we are unable to accept Posts that present what we consider to be free advertising for products that Harbeth does not make.

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{Updated Nov. 2016A}
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Early analogue recordings & an evaluation of analogue technology ...

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  • #31
    Analogue Anomalies Pt. 5 - what about music!

    What about music indeed.

    Just before I packed away the Revox for another few years I though I'd play some music and compare on and off tape. The very first music file I had to hand had a nice strong high frequency component, so that would do very nicely.

    You'll recall that I demonstrated the absolute curse of unwanted background noise. It covers-over all the fine tonal detail with a random spluttering and once masked by the noise, the fine tonal detail is irrecoverable. Because of the quirks of analogue (tape) and our hearing, the effect of the very poor frequency resolution is often masked in the middle frequencies, but as frequency increases, numerous additional limitations of analogue (tape) reveal themselves.

    By now you should appreciate that the vertical scale in the frequency spectral plot is signal loudness, in (arbitrary) dBs. We can slide the signal up and down that vertical scale just to give us a nice presentation. All that matters is the relative strength of frequency bands relative to others.

    If you listen carefully to this excerpt you can hear and see that the high-hat is not covered by the music and can sing out very naturally. The spectral display reveals that it has much energy above 10kHz, specifically above about 15kHz and extending right to the edge of the horizontal display - over 22kHz. We then record that high quality source onto tape and compare the on/off results. Whilst the frequency response at the top is quite similar in extension, note how the analogue off-tape signal does not ever drop down in level below about the arbitrary -84dB above about 15kHz. But the digital source drops to around -96dB. That means there is about 12dB more dynamic range in the high-hat reproduction than the analogue tape can reproduce. The problem was reveals in the previous episodes: tape hiss masking the quietest cymbal microtones. And this is revealed a slight softening of sound, which as we all know, some audio enthusiasts like.

    Here is >>> Analogue Anomalies Part 5 <<<

    Complete digital source and off-tape music tracks as follows both as moderate-rate MP3s. You decide which is which after considering the points covered in this thread about dynamic range, tape hiss, frequency response and resolution.

    Loading the player ...


    Loading the player ...


    Conclusion thus far: if you want to achieve more resolution from your high-fidelity replay system, the simplest, cheapest and easiest way to hear more of the microtones in music is to chose a replay system than has the lowest background noise be it noise in the medium (tapes, discs etc.) or in the driving equipment. But there is a practical limit of perfection. A continuation of my experiments here show that providing the dynamic range between the loudest sound reproducible and the background hiss is about 80dB or so no further improvement is audible. So chasing equipment with a noise floor at -100 or -120dB is pointless. So the real issue then is pushing down the noise floor between what we have seen is around -50dB in analogue tape (proximity of close tones) and the edge of audibility at around -80dB. That's really very difficult indeed for any analogue system to achieve.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


    • #32
      Off-tape sound fooled me?

      Well, I'll wager that the first of the two clips (A) was the off-tape sample - there was no significant loss of "sparkle", but the ride cymbal had slightly less attack than was evident in the second clip (B). There was also, surprisingly, a subtle difference in the sound of the brush work on the snare.

      A point of significance is how good a job the old lady (B77 analogue tape recorder) made of this recording after several years on ice and the obvious problems that showed up with the earlier multi-tone test.

      Having watched the Part 5 video, it is interesting how the direct recording shows an obvious steep low pass filter at about 15kHz (probably an artefact of the MP3 conversion) which was not so much in evidence with the off-tape sample. It's as though the MP3 engine has been "fooled" into thinking that it was all done by about 12kHz, only to "find" that there really was more up top that had to be acknowledged!

      Now if I had been asked to choose which of these two I might prefer from looking at the spectra alone, I'd probably have opted for the off-tape version without the cliff-edge dip at 15kHz.

      Comment


      • #33
        Are we starting to see more High Definition music become available?

        One of my favourite pieces of music is the Beecham recording of Peer Gynt made in stereo by EMI in 1957 - the year I was born. I had an old reel-to-reel tape back in the 60s, and now I have the CD. It still sounds fresh - and better than any other version I have heard.

        Back on an earlier comment about MP3 taking us backward - I personally am not too concerned - lossey music is just a short term by-product of storage constraints. We have already seen the default compression rate rise from 128 to 256 on iTunes and to 320kb/s on Amazon. Vendors are bringing more sophisticated rip and store devices to market that capture CD quality. And we are starting to see more High Definition music become available.

        Comment


        • #34
          Revised examples from post #31 - source and off tape

          Originally posted by Pluto View Post
          Well, I'll wager that the first of the two clips (A) was the off-tape sample - there was no significant loss of "sparkle", but the ride cymbal had slightly less attack than was evident in the second clip (B). There was also, surprisingly, a subtle difference in the sound of the brush work on the snare.....
          Actually Pluto, I just realised that I made a mistake presenting these examples and need to re-do them.

          You will recall that we were using the 'Monitor' switch on the front of the B77 to listen to the signal going into the recorder or in the other position, the signal that had just been recorded onto tape. The mistake I made was this: when the switch is in the 'input' position, we are not monitoring the signal directly at the input sockets on the recorder, we are monitoring the signal after it has passed through some amplification inside the Revox. I did know this because if you look carefully on any of the videos you will see that there are second/third/fourth etc. harmonic tones present in the 'input' position which are not there on the source but I forgot to consider this when presenting the music examples. Harmonic distortion is evidence that some amplification (and not very good amplification) is in the signal path. For the purpose of all the demonstrations I made, this is irrelevant because the dominant problems are from the tape itself, not the analogue electronics inside the Revox.

          But when we come to compare the first generation 'source' recording with the off tape, we must directly compare the source with the off tape and not rely on the Monitor switch on the Revox.

          So here we are again, this time with the first generation digital sound and the off tape. This is a true comparison of the effect of passing a digital recording through an analogue tape recorder. I have cut-off the lineup tone used to equalise the levels.

          Loading the player ...
          Version C

          Loading the player ...
          Version D
          Alan A. Shaw
          Designer, owner
          Harbeth Audio UK

          Comment


          • #35
            Learning from analogue and applying to MP3

            It's interesting how one idea leads to another.

            My guess would be that those clever folks who designed the MP3 coding system thoroughly understood the characteristic of analogue media. They would, for sure, have known how quite tones are masked by louder ones. My guess is that their starting point would be to devise a data-reduction process which mimics the noise masking process that we have seen applies to analogue tape.

            If that is so, it should be easy to see noise profiling in the MP3 encoding system. More information to follow after some experiments (in hand).
            Alan A. Shaw
            Designer, owner
            Harbeth Audio UK

            Comment


            • #36
              Transistors v. ICs

              Originally posted by A.S. View Post
              Actually Pluto, I just realised that I made a mistake presenting these examples and need to re-do them.
              OK, this time item C is off-tape - the difference is rather more obvious this time (which makes it easier, assuming I was correct last time!!).

              OTOH it doesn't show the Revox electronics in a very good light; either the Old Lady really does need a full electronic overhaul (probably duff capacitors after all this time) or the sound quality of cheap electronics really has progressed after thirty years. Maybe both are true.

              I get rather annoyed with those who allege that IC op-amps, in general, 'degrade' the sound* - the concept of demonstrable transparency doesn't mesh easily with the audiophile creed.

              * the point being that the Revox A/B77 electronic design precedes the widespread use of op-amps because, at that time, they were NOT good enough. These days, entirely the reverse is true.

              Comment


              • #37
                Revox revoiced?

                Another thing springs to mind in the light of your revelation about the switching: are you sure about the Revox response plot shown briefly in the video? While I didn't notice the graph scaling, I remember thinking that it was either extraordinarily good or not actually off tape! What tape speed was running?

                Comment


                • #38
                  Originally posted by A.S. View Post
                  It's interesting how one idea leads to another. My guess would be that those clever folks who designed the MP3 coding system thoroughly understood the characteristic of analogue media. They would, for sure, have known how quite tones are masked by louder ones. My guess is that their starting point would be to devise a data-reduction process which mimics the noise masking process that we have seen applies to analogue tape...
                  And indeed, if we make a few simple experiments we can see that is exactly the line of thinking that led from analogue to pure digital and then to analogue-like (MP3) data compressed audio. It's so blindingly obvious that I don't know why it hadn't occurred to me before.

                  Fortunately I have already covered the basics of how the noise floor of analogue tape (unlike digital) is elevated in the proximity of recorded tones in the previous Analogue Anomalies videos a few posts back. This noise floor variability has something to do with the fact that in analogue tape, the sound is actually converted to magnetism, and stored in tiny bar magnets glued to the plastic tape. Those particle magnets do not exist in isolation (they are heaped up on one another) and hence, unlike a digital 'bit' which is uniquely addressable and turned off or on, they will be influenced by the general magnetic field in their proximity. And if there is a loud signal (therefore a strong magnetic field) that field will bleed into adjacent magnetic particles of the tape and will certainly swamp (or even actually erase) weaker signals. We've seen how the noise floor ramps-up in the vicinity of tones. We have also looked at how there tiny magnets have sufficient stray field that they can print-through onto adjacent layers (see a few videos back) so they are quite strong.

                  Well, guess what. Armed with what we've discovered about analogue tape we can now use the same method to evidence that MP3 uses fundamentally the same 'encoding' process as analogue tape. In the absence of any tones or music, that is, recording silence, MP3 as is typical of any good digital system, and has a very low background noise level. In fact, when recording silence we'll see that the MP3 system has the exceptionally low background hiss level of the best pure-digital systems. But the really cunning thing about MP3 we will see is that the MP3 coder actually generates than cleverly manipulates a background shaped noise (i.e. frequency specific hiss) just as I did to show how the profile of analogue tapes noise could be added to the pure digital signal (a few videos back). The MP3 codec ingeniously actually introduces noise where there was no noise before. And we've seen in the previous video that no signal can be retrieved from under the hiss. Turning that around, we can say that if there is hiss of a certain quantity (loudness, level) there is absolutely no need for the MP3 encoder to even attempt to encode those tones that would be inaudible under the hiss. So, unlike the analogue tape recorder where all signals regardless of how loud or quiet, regardless of what frequency band they are in, all are presented to the tape record head (and only some are detectable off-tape as we've seen), the MP3 encoder actually makes second-by-second decisions based on the audio content which signals are louder than the hiss and worth encoding, and which ones to discard as swamped by the hiss. Very cunning indeed. Greatly reduces the data rate.

                  I'll make a video of this later.

                  UPDATE 1: After some experimentation, it's clear that the MP3 coder is far smarter than just a crude emulation of the characteristics of analogue recording tape. It has evident intelligence. It certainly does analyse the signal to be encoded and it seems to have, as I suggested, defined code/don't code loudness thresholds. The way it behaves if a signal is just a little above the threshold is very different to the behaviour just below the threshold, in the 'don't bother attempting to encode' area. But surprisingly it does introduce significant amounts of harmonic distortion - that is, the coder generates (2nd) harmonics that are completely absent in the source audio signal as indeed analogue tape does. Is this a deliberate strategy to 'warm-up' the sound?
                  Alan A. Shaw
                  Designer, owner
                  Harbeth Audio UK

                  Comment


                  • #39
                    Specially modded B77

                    I specially modded this B77 years ago to be damned nearly flat. 15 ips. I'll find the HFN article I wrote on it.

                    Note: This article is not on our server. That means it has not been scanned. I will find the original HFN and scan it asap.
                    Alan A. Shaw
                    Designer, owner
                    Harbeth Audio UK

                    Comment


                    • #40
                      Originally posted by Pluto View Post
                      Another thing springs to mind in the light of your revelation about the switching: are you sure about the Revox response plot shown briefly in the video? While I didn't notice the graph scaling, I remember thinking that it was either extraordinarily good or not actually off tape! What tape speed was running?
                      Actually Pluto - and I'm a bit surprised your golden-olden ears didn't detect this - the burbling in the lineup tone gave the game away as to which recording was, beyond a shadow of doubt, off tape!
                      Alan A. Shaw
                      Designer, owner
                      Harbeth Audio UK

                      Comment


                      • #41
                        Originally posted by A.S. View Post
                        I'm a bit surprised your golden-olden ears didn't detect this - the burbling in the lineup tone gave the game away...
                        I've listened to quite enough line up tone, thanks - I whizz through it these days

                        Actually, the LF burbling was more obvious on the decay at the very end of the off-tape clip, now that I've actually listened to it all the way through (too impatient, that's my trouble).

                        If that really is the off-tape response of the Revox, very impressed. Also with the fact that you have a still viable supply of Type 200 - exactly which version? The best one was probably Agfa PEM something-or-other and there was also Zonal - rather more variable in sensitivity.

                        Comment


                        • #42
                          Type 200 lives - or exists anyway - at Harbeth

                          Originally posted by Pluto View Post
                          ...If that really is the off-tape response of the Revox, very impressed. Also with the fact that you have a still viable supply of Type 200 - exactly which version? The best one was probably Agfa PEM something-or-other and there was also Zonal - rather more variable in sensitivity.
                          Surely you don't want me to turn the video onto the switch and watch the input/off tape action do you!

                          Plenty of *brand spanking new* Type 200 here on 5" and 10" reels. You're alluding to BASF SM468 and Zonal 675. The sensitivity of the Zonal is slightly lower.
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • #43
                            Revox mods

                            Originally posted by A.S. View Post
                            Surely you don't want me to turn the video onto the switch and watch the input/off tape action do you?
                            No - I trust you - but I would be interested in seeing the HFN article. What was the essence of the modification to get it that flat?

                            The remarkable thing about the tape is not that you have it, but that it remains usable without shedding the oxide.

                            Comment


                            • #44
                              Modifications to the Revox B77 to make flat replay

                              Originally posted by Pluto View Post
                              ...but I would be interested in seeing the HFN article. What was the essence of the modification to get it that flat?
                              As I recall, I considered modifying both the record EQ and the replay EQ. The normal attack on designing tape recorder electronics is to standardise the replay EQ so that a known calibration tape will replay with the correct frequency response regardless of the machine it is replayed on and to adjust the record EQ by whatever means and manner is expedient to make a tape recorded on that machine (after aligning the replay performance) replay as flat.

                              In the case of the Revox, in comparison with the much more adjustable Studers, there are no user adjustments (if I recall) for record or replay EQ. So I modelled in software and then reworked the record electronics to increase the drive to the head at audio HF a little, so that the record/replay result was flat.

                              This B77 was made with NAB standard electronics (and still has) but the CCIR/IEC standard is used in the BBC and on my Studers. My AEG Telefunken M21 is programmable for either across a wide range of speeds up to and including 30 ips.: very sophisticated design.

                              The M21 weighs 45kg despite its compact size and it just wasn't possible to bring it into my den. Literature attached. Mine is like the reverse-head unit shown on the cover: that means the magnetic oxide side of the tape must be outwards, not inwards. I believe this is popular in Europe.

                              >
                              Attached Files
                              Alan A. Shaw
                              Designer, owner
                              Harbeth Audio UK

                              Comment


                              • #45
                                Dolby noise reduction

                                Yes, having recorded 400 Hz Dolby tones, the burbling was a dead give away. Speaking of Dolby wasn't the use of the professional "A" and consumer "B&C" quite common in the later days to improve the signal to noise ratios?

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