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ARSCLIST  April 2012

ARSCLIST April 2012

Subject:

Re: ARSCLIST Digest - 13 Apr 2012 to 14 Apr 2012 (#2012-101)

From:

Shai Drori <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Sun, 15 Apr 2012 18:51:29 +0200

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (260 lines)

Exactly what I said RE a77. Or was that in a private mail?
Shai

בתאריך 04/15/12 3:49 PM, ציטוט Art Shifrin:
> Hi Gang,
>
> Regarding  "Slow Reel-to-reels" , vari--speed does NOT due the rest.  Heads
> with particularly narrow gaps should be used to read snail-pace tapes.
> Having that extra resource permits
> better extraction of highs than does pedal to the metal eq. adjustments.
>
> JRF has made superb specials for me for my repro-only AG 440.
>
> Best Regards,
> Art (Shiffy) Shifrin
>
>
> On Sun, Apr 15, 2012 at 12:00 AM, ARSCLIST automatic digest system<
> [log in to unmask]>  wrote:
>
>> There are 4 messages totalling 241 lines in this issue.
>>
>> Topics of the day:
>>
>>   1. Slow Reel-to-reels
>>   2. Recording_78rpm_records/analog vs digital eq (3)
>>
>> ----------------------------------------------------------------------
>>
>> Date:    Sat, 14 Apr 2012 10:11:02 +0200
>> From:    Shai Drori<[log in to unmask]>
>> Subject: Re: Slow Reel-to-reels
>>
>> Stock machine goes down to 3.75. Varispeed does the rest. At these
>> speeds I use my ears for eq more than the set standards. The key is the
>> gap of the head.
>> Shai
>>
>> בתאריך 04/13/12 10:14 PM, ציטוט Stephen Bolech:
>>> Shai, did you have it modified to play at those speeds, or do they have
>> that as an option?
>>>
>>> On Apr 13, 2012, at 2:59 PM, Shai Drori wrote:
>>>
>>>> I use the Ampex atr-100
>>>> Shai Drori
>>>>
>>>>
>>>> בתאריך 04/13/12 8:57 PM, ציטוט Stephen Bolech:
>>>>> Hi everyone, I'm hoping some of you could give me recommendations for
>> good options to play back 1 7/8 ips and even the occasional 15/16 tapes.
>>   We have a large oral history collection, and though the majority are at
>> 3.75 ips, there are some at these slower speeds.  What are you guys using
>> for these speeds, and what do you recommend?
>>>>> Thank you,
>>>>> Stephen Bolech
>>>>>
>>>> --
>>>> בברכה,
>>>> שי דרורי
>>>> מומחה לשימור והמרה של אודיו וידאו וסרטים 8-35 ממ.
>> --
>> בברכה,
>> שי דרורי
>> מומחה לשימור והמרה של אודיו וידאו וסרטים 8-35 ממ.
>>
>> ------------------------------
>>
>> Date:    Sat, 14 Apr 2012 10:06:58 +0000
>> From:    Don Cox<[log in to unmask]>
>> Subject: Re: Recording_78rpm_records/analog vs digital eq
>>
>> On 13/04/2012, George Brock-Nannestad wrote:
>>
>>
>>> The so-called Nyquist criterion states that if you want to be able to
>>> reconstruct a signal from a sampled representation of it, you have to
>>> decide the highest frequency you want represented and then you have to
>>> sample at a rate that is at least twice the highest frequency.
>> Note, _at least_. It won't do any harm to go higher.
>>
>>> When
>>> you then reconstruct your signal by providing voltages at the sample
>>> rate according to the table of values that represents the audio
>>> signal, you will have your full bandwidth and dynamic range back,
>>> provided the resolution or bit-depth has been sufficient. It has been
>>> claimed that this stepwise presented waveform makes the signal
>>> unlistenable, but that is not the case, because you invariably smooth
>>> it by filtering, so nothing above your defined highest frequency gets
>>> out in the analog domain again. You will note that you are in full
>>> control: define the maximum frequency and define the resolution. If
>>> they are not sufficient to your purpose, go higher.
>>>
>>> Now, we have a problem with pure sampling of a waveform: if it has a
>>> frequency that is more than half of the sampling rate, that too will
>>> be sampled, but in this case under-sampled, which means that the
>>> result appears to be at a completely different and inharmonic and
>>> jarring frequency, an alias. Once we have adopted a sampling frequency
>>> we must simply ensure that no signal above half the sampling rate is
>>> available for sampling.
>>>
>> It is interesting to see the digital photography community wrestling
>> with these concepts. (Most cameras use optical anti-aliasing filters.)
>>
>>> This is done by filtering, so-called anti-alias filtering. With 44.1
>>> kHz sampling rate, no signal above 22.05 kHz is permissible. On the
>>> other hand, we do want our 20 kHz bandwidth - this is what a young,
>>> pre-earbud ear can mostly hear.
>> There seems to be good evidence that we can hear timing differences on
>> impulse signals corresponding to much higher frequencies than the
>> continuous sine waves normally used for testing hearing.
>>
>>> So, our filter has to go from full
>>> transmission at 20kHz to zero transmission at 22.05 kHz *). No
>>> problem, our telephone engineers have done this kind of exercise for
>>> 90 years. However, such sharp cut-off filters come with some frightful
>>> time delay distortion (phase to some), very audible down to 2 kHz.
>>> That was the situation for about 10 years in CD audio, until someone
>>> came up with the idea to correct the time response in the digital
>>> domain by means of a digital filter. The currency to pay for this is
>>> total delay time, but for something recorded a year ago, some
>>> microseconds do not matter.
>>>
>>> In other words, many of our problems come from the filter. If we
>>> increase the sampling rate we can make use of our more frequent
>>> samples in two ways: we can do a gentler filtering that does not have
>>> the delay effect at low frequencies, or we can increase our highest
>>> frequency. If we do the latter, we shall be able to increase the time
>>> resolution of our digital representation. With 20 kHz the maximum
>>> slope of the waveform is only one quarter of that of an 80 kHz
>>> bandwidth - reachable by a 192 kHz sampling frequency and a gentler
>>> anti-aliasing filter.
>>>
>>> A couple of other items came up while we were at 78rpm reproduction:
>>>
>>> - the reason why we need an elevated bandwidth for recordings on
>>> rough surfaces is because that is where the noise signal is.
>> This is the crucial point.
>>
>>
>>
>>> - one might consider that the pickup would be encountering a
>>> formidable vertical wall when it met a square wave. However, the
>>> recording is usually a velocity recording, that is the square
>>> waveshape is differentiated (1st derivative), which means it turns
>>> into a triangular wave. The problem is that with a high bandwidth, the
>>> corners of this triangular wave (where the square shifts from constant
>>> positive to constant negative and vice versa) are very sharp, and even
>>> that may be difficult to trace. George Alexandrovich made a 7" test
>>> record with a "square wave" - there are virtually no wiggles at the
>>> corners of the triangles. It is for testing pickups, but I have not
>>> dared to use it on anything but the ELP.
>>>
>> Back in the days when The Gramophone carried technical reviews of
>> equipment, under John Borwick especially, oscilloscope traces of pickup
>> cartridges playing "square waves" were shown in all reviews. I assume
>> the actual waveforms on the LP were triangular.
>>
>>
>>> - for the same reason a tick, modelled by a steep rise followed by
>>> steep fall becomes two ticks, one positive and one negative.
>>>
>>> - even if you remove these two peaks and interpolate or draw a
>>> waveform connecting the ends, the low frequency excitation of the
>>> whole cartridge-tonearm system (stylus, cantilever, bearing, cartridge
>>> mass, tonearm mass and resonances) still remains as a low-level thud -
>>> a tail. CEDAR started back in 1988 with a program developed by Peter
>>> Rayner while still at the British Library to remove not only the
>>> ticks, but also the tail.
>>>
>> Regards
>> --
>> Don Cox
>> [log in to unmask]
>>
>> ------------------------------
>>
>> Date:    Sat, 14 Apr 2012 11:01:20 -0400
>> From:    "Richard L. Hess"<[log in to unmask]>
>> Subject: Re: Recording_78rpm_records/analog vs digital eq
>>
>> Hi, Don,
>>
>> In the early 1970s, I recall getting into a long, and semi-heated
>> discussion between two brands of cartridges. My colleague had done scope
>> photos of various cartridges reproducing CBS Labs test records,
>> including square waves. He said one cartridge was better than my
>> favourite because it didn't show the ringing in the square waves. I said
>> I didn't like his cartridge because it sounded dull and lifeless
>> compared to my favourite which sounded more like live music.
>>
>> I also recall, a half decade or more later, a magazine, I think it was
>> AUDIO Magazine, showed scanning electron micrographs  (SEM images) of
>> the grooves of one or more CBS Labs test records and the ringing showed
>> up in the SEM images!
>>
>> Cheers,
>>
>> Richard
>>
>> On 2012-04-14 6:06 AM, Don Cox wrote:
>>> Back in the days when The Gramophone carried technical reviews of
>>> equipment, under John Borwick especially, oscilloscope traces of pickup
>>> cartridges playing "square waves" were shown in all reviews. I assume
>>> the actual waveforms on the LP were triangular.
>> --
>> Richard L. Hess                   email: [log in to unmask]
>> Aurora, Ontario, Canada           (905) 713 6733     1-877-TAPE-FIX
>> http://www.richardhess.com/tape/contact.htm
>> Quality tape transfers -- even from hard-to-play tapes.
>>
>> ------------------------------
>>
>> Date:    Sat, 14 Apr 2012 16:54:15 +0000
>> From:    Don Cox<[log in to unmask]>
>> Subject: Re: Recording_78rpm_records/analog vs digital eq
>>
>> On 14/04/2012, Richard L. Hess wrote:
>>
>>> Hi, Don,
>>>
>>> In the early 1970s, I recall getting into a long, and semi-heated
>>> discussion between two brands of cartridges. My colleague had done
>>> scope photos of various cartridges reproducing CBS Labs test records,
>>> including square waves. He said one cartridge was better than my
>>> favourite because it didn't show the ringing in the square waves. I
>>> said I didn't like his cartridge because it sounded dull and lifeless
>>> compared to my favourite which sounded more like live music.
>>>
>>> I also recall, a half decade or more later, a magazine, I think it was
>>> AUDIO Magazine, showed scanning electron micrographs (SEM images) of
>>> the grooves of one or more CBS Labs test records and the ringing
>>> showed up in the SEM images!
>>>
>> That is funny. Make sure you are testing for what you think you are
>> testing for.  ;-)
>>
>> I would like to see those images.
>>
>>> On 2012-04-14 6:06 AM, Don Cox wrote:
>>>> Back in the days when The Gramophone carried technical reviews of
>>>> equipment, under John Borwick especially, oscilloscope traces of
>>>> pickup cartridges playing "square waves" were shown in all reviews. I
>>>> assume the actual waveforms on the LP were triangular.
>> Regards
>> --
>> Don Cox
>> [log in to unmask]
>>
>> ------------------------------
>>
>> End of ARSCLIST Digest - 13 Apr 2012 to 14 Apr 2012 (#2012-101)
>> ***************************************************************
>>

-- 
בברכה,
שי דרורי
מומחה לשימור והמרה של אודיו וידאו וסרטים 8-35 ממ.

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