New page on Squares waves and amplifier performance
 

"bcoombes" bcoombes@orangedotnet wrote in message
o.uk...
bcoombes wrote:
David Looser wrote:
"bcoombes" bcoombes@orangedotnet wrote in message
...
Bill Taylor wrote:
On Sun, 10 Jan 2010 10:22:12 +0000, bcoombes bcoombes@orangedotnet
wrote:

The cassette deck might have sliced everything above 8k off, but Dolby
B didn't.
But on these decks it did because with the Dolby off there was response
up to at least 12k whereas with it on it didn't just remove the hiss it
removed everything..as per my not properly implemented comment.


What tolerance was there on the response up to12k?, and at what rate did
the response fall off above 8k with Dolby on? What level, relative to
Dolby level, did you make your measurements at?


Hardly necessary since what was missing was/is clearly audible. As it
happens I still have a Panasonic RQ-X20D cassette player which I've just
dug out of storage and had a listen to to confirm the effect and it's
totally obvious, with the Dolby on some of the 'music' is just plain
MISSING. My original 8k figure was a [mis]remembered estimate, it
actuallity it must be lower than that.
ISTR that Panasonic were established to be one of the manufacturers that
took a while to get the *new* Dolby technology right but the defect was
quite common for a while and was often commented on in reviews of
cassette decks of the time.

On the above subject here's some technical stuff on Dolby NR
implementation problems. No doubt many of the early and 'cheap' cassette
decks *incorporated* them but it seems it may also be a problem with the
recordings. Any comments from some of the people here who obviously know
about this stuff gratefully received since I'm more interested in learning
than 'being right'.

1. Pumping:
Incorrect selection of the control path bandwidth external components can
result in an audible increase in noise as the input level changes. This is
most likely to be heard on solo instruments or on speech. Sometimes the
S/N rate is too poor and masking will not be completely effective - i.e.,
when the bandwidth is wide enough to pass the program material, the
increase in noise is audible. Cutting down on the pumping will also affect
the program material to some extent and judgement as to which is
preferable is required. Sometimes a shorter decay time constant in the
detector circuit will help, especially for a source which always shows
these characteristics, but for better program material a return to the
recommended detector characteristics is imperative.
2. High Frequency Loss:
This can be caused by an improper control path gain setting-perhaps
deliberate because of the source S/N ratio as described above-or incorrect
values for the
audio path filter capacitors. Capacitors larger than the recommended
values will scale the operating bandwidth lower, causing lower -3 dB
corner frequencies for a given control path signal. Return to the correct
capacitor values and the appropriate control path gain setting will always
ensure that the h.f. content of the signal source is preserved.


In the mid 80s, I owned a HiFi shop, and sold a lot of Nakamichi machines.
As part of the sale, I offered to line up the machine to the users'
preferred tapes. Few users understood that tape formulations differ, even
between batches of the nominally same tape. Nakamichis were truly excellent
machines, and were capable of exceptional performance, but, they had to be
immaculately lined up. Cassette generally is at the limit of what is
possible, and Nakamichis were at the limit of the limit.

Nevertheless, a machine could be lined up to give a 30Hz-20kHz -2dB
response, with a 60dB S/N ratio, well under 0.1% W&F and less than 3%
distortion at 1kHz and 0dB where 0dB was 200mM/mm all simultaneously using
Dolby C on metal tape, and with Dolby tracking to within 1dB. The big
however, was that changing tapes to another metal tape, say TDK from Maxell,
or That's (my favourite) to TDK would destroy the performance. Frequency
response, Dolby tracking, distortion, noise, would all get significantly
worse. Then the tape manufacturers wouldn't help by constantly "upgrading"
their tapes which meant that the machine's record side had to be lined up
all over again. I used to encourage customers to buy a couple of boxes of
tapes with their machine, to give them a fighting chance of having a stock
of tapes the machine was aligned for.

I often used to hear that Dolby was awful, that people preferred to record
without Dolby and play back with, or the other way round, that B was much
better than C etc etc. C was a LOT better than B, but was correspondingly
more critical of line-up (Dolby C was effectively two Dolby B processors in
series..the first implementation of Dolby C was actually that, then a
dedicated B/C chip became available which unfortunately wasn't totally
compatible with the first implementation. All the horror stories of Dolby
essentially came down to lack of alignment...you can't expect the Public to
line up their machine every time they used it, but that's pretty much what
it took if you wanted A-B transparency, which is what a freshly lined-up
Nakamichi could do. Few other machines gave you access to record EQ, record
and bias current, replay EQ, separate record and playback heads, head
positioning, dual capstans that took away the cassette's pressure pads etc,
and so their line-up was always compromised, if it ever happened.

With my own Nakamichi, I would do a full line-up every time I wanted to do a
"proper" recording, i.e. one to keep, but it became a real fag, and wasn't
sad to give up tape for PC recording.

S.

New page on Squares waves and amplifier performance
 

Serge Auckland wrote:

"bcoombes" bcoombes@orangedotnet wrote in message


ISTR that Panasonic were established to be one of the manufacturers
that took a while to get the *new* Dolby technology right but the
defect was quite common for a while and was often commented on in
reviews of cassette decks of the time.

On the above subject here's some technical stuff on Dolby NR
implementation problems. No doubt many of the early and 'cheap'
cassette decks *incorporated* them but it seems it may also be a
problem with the recordings. Any comments from some of the people here
who obviously know about this stuff gratefully received since I'm more
interested in learning than 'being right'.

1. Pumping:
Incorrect selection of the control path bandwidth external components
can result in an audible increase in noise as the input level changes.
This is most likely to be heard on solo instruments or on speech.
Sometimes the S/N rate is too poor and masking will not be completely
effective - i.e., when the bandwidth is wide enough to pass the
program material, the increase in noise is audible. Cutting down on
the pumping will also affect the program material to some extent and
judgement as to which is preferable is required. Sometimes a shorter
decay time constant in the detector circuit will help, especially for
a source which always shows these characteristics, but for better
program material a return to the recommended detector characteristics
is imperative.
2. High Frequency Loss:
This can be caused by an improper control path gain setting-perhaps
deliberate because of the source S/N ratio as described above-or
incorrect values for the
audio path filter capacitors. Capacitors larger than the recommended
values will scale the operating bandwidth lower, causing lower -3 dB
corner frequencies for a given control path signal. Return to the
correct capacitor values and the appropriate control path gain setting
will always ensure that the h.f. content of the signal source is
preserved.


In the mid 80s, I owned a HiFi shop,

Where was it and what was it's name..being nosy. :)

and sold a lot of Nakamichi
machines. As part of the sale, I offered to line up the machine to the
users' preferred tapes. Few users understood that tape formulations
differ, even between batches of the nominally same tape. Nakamichis were
truly excellent machines, and were capable of exceptional performance,
but, they had to be immaculately lined up. Cassette generally is at the
limit of what is possible, and Nakamichis were at the limit of the limit.

Nevertheless, a machine could be lined up to give a 30Hz-20kHz -2dB
response, with a 60dB S/N ratio, well under 0.1% W&F and less than 3%
distortion at 1kHz and 0dB where 0dB was 200mM/mm all simultaneously
using Dolby C on metal tape, and with Dolby tracking to within 1dB. The
big however, was that changing tapes to another metal tape, say TDK from
Maxell, or That's (my favourite) to TDK would destroy the performance.
Frequency response, Dolby tracking, distortion, noise, would all get
significantly worse. Then the tape manufacturers wouldn't help by
constantly "upgrading" their tapes which meant that the machine's record
side had to be lined up all over again. I used to encourage customers
to buy a couple of boxes of tapes with their machine, to give them a
fighting chance of having a stock of tapes the machine was aligned for.

With technology comes incompatability problems it seems. :)
Doesn't sound like much of an improvement over vinyl in 'faffing about' terms.

I often used to hear that Dolby was awful, that people preferred to
record without Dolby and play back with, or the other way round, that B
was much better than C etc etc. C was a LOT better than B, but was
correspondingly more critical of line-up (Dolby C was effectively two
Dolby B processors in series..the first implementation of Dolby C was
actually that, then a dedicated B/C chip became available which
unfortunately wasn't totally compatible with the first implementation.

Very interesting, I had a cruise around the Dolby site (and others) and didn't
see any mention of that...unsurprisingly. :)

All the horror stories of Dolby essentially came down to lack of
alignment...you can't expect the Public to line up their machine every
time they used it, but that's pretty much what it took if you wanted A-B
transparency, which is what a freshly lined-up Nakamichi could do. Few
other machines gave you access to record EQ, record and bias current,
replay EQ, separate record and playback heads, head positioning, dual
capstans that took away the cassette's pressure pads etc, and so their
line-up was always compromised, if it ever happened.

ISTR that even Nakamichi car decks outperformed other manufacturers 'hi-fi
separate' decks, they used to cost an arm and a leg AIR.

With my own Nakamichi, I would do a full line-up every time I wanted to
do a "proper" recording, i.e. one to keep, but it became a real fag, and
wasn't sad to give up tape for PC recording.

Yeah with pc's everyone's a producer (for good or bad) as they say. :)
Fascinating post..thanks.


--
Bill Coombes

New page on Squares waves and amplifier performance
 

"Serge Auckland" wrote


With my own Nakamichi, I would do a full line-up every time I wanted to do
a "proper" recording, i.e. one to keep, but it became a real fag, and
wasn't sad to give up tape for PC recording.

My Aiwa has auto bias and eq., and it works too! Press a button and the
machine optimises first bias, then eq and finally level for the actual tape
that's in the machine. In other respects too it's good machine, double
capstan drive ensure low W&F and good head-tape contact. Recordings made
onto a good tape formulation are impressive.

Nevertheless I still prefer not to use NR. Frankly it's unnecessary, with a
good type-2 tape formulation the little bit of tape hiss is hardly audible,
yet even when the tape recorder is lined-up as good as it can be Dolby B is
*still* audible in operation. I found the same using Dolby B with a
carefully lined-up Revox at 7.5in/sec.

David.

New page on Squares waves and amplifier performance
 

David Looser wrote:
"bcoombes" bcoombes@orangedotnet wrote
Hardly necessary since what was missing was/is clearly audible. As it
happens I still have a Panasonic RQ-X20D cassette player which I've just
dug out of storage and had a listen to to confirm the effect and it's
totally obvious, with the Dolby on some of the 'music' is just plain
MISSING. My original 8k figure was a [mis]remembered estimate, it
actuallity it must be lower than that.
ISTR that Panasonic were established to be one of the manufacturers that
took a while to get the *new* Dolby technology right but the defect was
quite common for a while and was often commented on in reviews of cassette
decks of the time.

What you were hearing was dynamic expansion of the HF. The HF isn't
"missing" exactly, but it has been pushed down in level by up to 10dB,

Yeah that's right, it was only half as loud, maybe my choice of 'sliced' to
describe the effect was a bit OTT but it's the pushed down level wot I meant guv.

depending on it's frequency and on it's original level. The audible effect
is unpleasant IMO,

I quite agree, that's why I remembered it after all this time. :)

probably subjectively worse than the simple HF roll-off
you took it to be.

And I said something about HF roll-off where exactly?

I long ago gave up using Dolby NR on cassettes; I have a fairly up-market
Aiwa with auto bias and eq adjustment, but even with that I preferred to
sound with Dolby off. A little bit of tape hiss is pretty innocuous, far
less subjectively disturbing than hearing NR systems at work.


Currently I have a Denon 540 which sounds great to my (admittedly f***ed) hearing.

--
Bill Coombes

New page on Squares waves and amplifier performance
 

"bcoombes" bcoombes@orangedotnet wrote

And I said something about HF roll-off where exactly?

quote
It solved the hiss problem by simply slicing off everything above 8k.
unquote

David.

New page on Squares waves and amplifier performance
 

"Don Pearce" wrote in message
...
On Mon, 11 Jan 2010 18:19:50 -0000, "David Looser"
wrote:


It may have been "unheard of", but it existed all the same. Any amplifier,
with or without feedback, can exhibit slew-rate limiting if the bandwidth
is
insufficient to cope with the rate of rise or fall of the input signal.

Excessive bandwidth is the cause of the problem.

You mean excessive bandwith of the input signal I presume. That's another
way of saying insufficient bandwidth in the amplifier.

Slew rate limiting is
in fact plain ordinary limiting (sawing the tops off a sine wave) but
in the current domain when feeding a capacitor. Because of the
differentiation it looks in the voltage domain like a straight slop.

That's one mechanism, there are others, the Miller effect for example.

Excessive bandwidth permits large fast signals that will show limiting
of this kind.

Indeed, excessive bandwidth of the incoming signal. But as Jim showed a
simple passive filter on the input to the amplifier solves it.

David.

New page on Squares waves and amplifier performance
 

David Looser wrote:

When valve amps were the norm, true "slew rate limiting"
was unheard of, AFAIK. Not enough gain or feedback.

It may have been "unheard of", but it existed all the
same. Any amplifier, with or without feedback, can exhibit
slew-rate limiting if the bandwidth is insufficient to
cope with the rate of rise or fall of the input signal.

Staggered. I really don't know what to say...where could I
begin?

One effect that I have seen referred to as slew rate
limiting can happen when a cathode follower with
inadequate bias current fails to keep up with the demands
of a capacitive load. Like real slew rate limiting, it is
an effect of feedback, and results when the triode
approaches turn-off only when the signal is both high
amplitude and high frequency...which is when the slew
rate is highest. However, for small amplitudes, the same
CF wouldn't suffer from the same problem even if the
input slew rate were the same.

Nope, CF failure can occur even with small amplitude
signals. The cathode voltage can only fall at the rate
determined by the time constant of the cathode resistor
and load capacitance. If the input falls faster than this
the output will not follow regardless of the signal
amplitude. CF failure can be a problem with video signals;
to drive a high-capacitance load with a CF may require a
load resistor so low in value as to represent a real
problem. In the original TV transmitter at Alexandra
Palace the cathode resistor of the CF at the output of the
modulation amplifier required water-cooling as it
dissipated over a kilowatt. Since the valve was a DH type
the filament supply for it came from a motor-generator set
mounted on tall insulators to minimise it's capacitance to
earth! More recent TV transmitter design has used White
cathode-followers or other forms of push-pull drive for
this function to reduce the power dissipation. The
transmitter I am currently working on uses 4 PL38s in a
White cathode-follower.


Staggered, once again. Next time I see you, you should be in
sackcloth and ashes.

In the meantime I suggest you check out how a cathode
follower works, and in particular how its output impedance
is defined.

Then look into the meaning of the phrase "slew rate
limiting". I guess Wikipedia would be an appropriate place
to start. You may argue that it has acquired a wider, more
sloppily-defined meaning, but I prefer to keep the useful
distinctions that a disciplined use of language is able to
convey. In any case, no matter how sloppily defined it may
have become, your own interpretation is far beyond the pale.

Looking at Wiki myself, note the reference to op amps, high
transconductance, and the implication of the presence of
feedback because of the role of the compensation capacitor.

Engineers weren't daft in the days of valve amps. Had slew
rate limiting been a problem, it would have been recognised.
Yes there are various forms and causes of current limiting,
which may appear similar to slew rate limiting, but that's
not what they are.

Ian

New page on Squares waves and amplifier performance
 

On Mon, 11 Jan 2010 19:51:04 -0000, "David Looser"
wrote:

"Don Pearce" wrote in message
...
On Mon, 11 Jan 2010 18:19:50 -0000, "David Looser"
wrote:


It may have been "unheard of", but it existed all the same. Any amplifier,
with or without feedback, can exhibit slew-rate limiting if the bandwidth
is
insufficient to cope with the rate of rise or fall of the input signal.

Excessive bandwidth is the cause of the problem.

You mean excessive bandwith of the input signal I presume. That's another
way of saying insufficient bandwidth in the amplifier.


Excessive bandwidth of the signal reaching the dominant pole in the
amplifier - that is cured by reducing the bandwidth of the amplifier
prior to that point. A single pole exactly counters the rising current
that causes slew rate limiting. You cannot dictate the properties of
the incoming signal - you must make sure that the amplifier has a
bandwidth that does not exceed the maximum slew rate.

Slew rate limiting is
in fact plain ordinary limiting (sawing the tops off a sine wave) but
in the current domain when feeding a capacitor. Because of the
differentiation it looks in the voltage domain like a straight slop.

That's one mechanism, there are others, the Miller effect for example.


The Miller effect is exactly what the dominant pole does - just at a
different scale. Charging current in the Miller capacitance is what
gets limited.

Excessive bandwidth permits large fast signals that will show limiting
of this kind.

Indeed, excessive bandwidth of the incoming signal. But as Jim showed a
simple passive filter on the input to the amplifier solves it.

Reducing the bandwidth of the amplifier.

d

New page on Squares waves and amplifier performance
 

"bcoombes" bcoombes@orangedotnet wrote in message
o.uk...
Serge Auckland wrote:
In the mid 80s, I owned a HiFi shop,

"bcoombes" bcoombes@orangedotnet wrote in message

Where was it and what was it's name..being nosy. :)



--
Bill Coombes

It was Beechwood Audio, opened in 1984 in Braintree Essex, then in 1986 in
Bury St Edmunds. Went bust in 1987 after failing to realise that very few
buyers of HiFi equipment had any engineering appreciation, and would buy
anything that the magazine scribblers recommended. I majored on CD when
every magazine was in the pay of the Flat Earth Society, nor would I have
any truck with stuff that didn't make sense from an engineering perspective.

Sold a lot of Quad, KEF and Nakamichi, but not enough to make it pay.

Ho Hum....
S.

New page on Squares waves and amplifier performance
 

David Looser wrote:

Indeed, excessive bandwidth of the incoming signal.

Excessive slew rate. There really is no other adequate way
of saying it. Not excessive bandwidth, or amplitude, or
combination of the two, but slew rate, pure and simple. A
signal of a particular frequency has a max slew rate which
depends on its amplitude, and vice versa, so neither
frequency nor amplitude alone define a slew rate. Although a
particular combination of frequency and amplitude defines a
maximum slew rate, it isn't a unique definition because it
could be the same for any number of other combinations and
waveforms.

But as Jim showed a simple passive filter on the input to
the amplifier solves it.

Not if it's bad enough so that it can occur at audio
frequencies, or frequencies otherwise necessary for the
satisfactory operation of the amp. Then the cause of the
limiting must be dealt with.

Ian