RCA Dynagroove - Exactly what is it?
 

"Don Pearce" wrote in message
...
On Sat, 19 May 2007 10:03:27 +0100, Jim Lesurf
wrote:

Your description would be more appropriate for a 'nulling' or cancellation
method where two sources are employed. But need not involve any feeback.
In the context of this thread, this distinction is relevant as the
'dynagroove' system was a method which tried to cancel or null one
distortion with another - but this involved *no* feedback mechanism
to compare the two in reality. The LP cutting system had no way to
sense what the LP replay system would actually do when playing the
LP.


This is pre-distortion. It is currently widely used in RF power
amplifiers to lift the third order intercept point sufficiently to get
acceptable output power from RF power transistors, which are notorious
for their "soft" transfer characteristic. GSM wouldn't be economically
possible without pre-distortion.

The transfer curve of the power amplifier is measured, and then one of
two things happens. If the solution is a hardware one, a matrix of
diodes and resistors is designed to straighten the overall curve.
Alternatively in software, the lookup table for the DAC that creates
the signal is changed to oppose the errors of the power amp.

d

--
Pearce Consulting
http://www.pearce.uk.com

Exactly the same procedure is used for high-power UHF Television
transmitters. Klystron distortion would be unacceptable without it, so Dif.
Phase and Dif. Gain are substantially reduced by pre-distorting the TV
signal. I remember one rather naughty incident when we had the Customer
doing Factory Acceptance, and we couldn't get both Dif Gain and Phase within
spec. at the same time, so the test engineers arranged for the Dif Gain
measurements to be made before lunch and Dif Phase after lunch. Whilst the
customer was having lunch, the Test Engineer changed the predistortion
characteristic so that both measurments would come out on spec. Not
surprisingly, the customer realised what was going on, red faces all round,
and sandwiches at the test bay rather than lunch thereafter.

S.

RCA Dynagroove - Exactly what is it?
 

On Sun, 20 May 2007 10:27:44 +0100, "Serge Auckland"
wrote:


"Don Pearce" wrote in message
...
On Sat, 19 May 2007 10:03:27 +0100, Jim Lesurf
wrote:

Your description would be more appropriate for a 'nulling' or cancellation
method where two sources are employed. But need not involve any feeback.
In the context of this thread, this distinction is relevant as the
'dynagroove' system was a method which tried to cancel or null one
distortion with another - but this involved *no* feedback mechanism
to compare the two in reality. The LP cutting system had no way to
sense what the LP replay system would actually do when playing the
LP.


This is pre-distortion. It is currently widely used in RF power
amplifiers to lift the third order intercept point sufficiently to get
acceptable output power from RF power transistors, which are notorious
for their "soft" transfer characteristic. GSM wouldn't be economically
possible without pre-distortion.

The transfer curve of the power amplifier is measured, and then one of
two things happens. If the solution is a hardware one, a matrix of
diodes and resistors is designed to straighten the overall curve.
Alternatively in software, the lookup table for the DAC that creates
the signal is changed to oppose the errors of the power amp.

d

--
Pearce Consulting
http://www.pearce.uk.com

Exactly the same procedure is used for high-power UHF Television
transmitters. Klystron distortion would be unacceptable without it, so Dif.
Phase and Dif. Gain are substantially reduced by pre-distorting the TV
signal. I remember one rather naughty incident when we had the Customer
doing Factory Acceptance, and we couldn't get both Dif Gain and Phase within
spec. at the same time, so the test engineers arranged for the Dif Gain
measurements to be made before lunch and Dif Phase after lunch. Whilst the
customer was having lunch, the Test Engineer changed the predistortion
characteristic so that both measurments would come out on spec. Not
surprisingly, the customer realised what was going on, red faces all round,
and sandwiches at the test bay rather than lunch thereafter.

Naughty! I'm surprised you weren't slung out...

I remember those days. I could grade a picture within 1 place on the
CCIR scale at a glance. I designed a microwave TV distribution system
for Hong Kong that used a waveguide multiplexer to combine thirty
channels (this was FM, like satellite, not VSB AM). Getting the phase
response of those flat enough to maintain diff phase was tricky. The
2T pulse was the hard one though.

I can't grade digital pictures by eye. I haven't seen one that I
wouldn't classify as "Not Received".

d

--
Pearce Consulting
http://www.pearce.uk.com

RCA Dynagroove - Exactly what is it?
 

In article , Serge Auckland
writes

"Don Pearce" wrote in message
...
On Sat, 19 May 2007 10:03:27 +0100, Jim Lesurf
wrote:

Your description would be more appropriate for a 'nulling' or cancellation
method where two sources are employed. But need not involve any feeback.
In the context of this thread, this distinction is relevant as the
'dynagroove' system was a method which tried to cancel or null one
distortion with another - but this involved *no* feedback mechanism
to compare the two in reality. The LP cutting system had no way to
sense what the LP replay system would actually do when playing the
LP.


This is pre-distortion. It is currently widely used in RF power
amplifiers to lift the third order intercept point sufficiently to get
acceptable output power from RF power transistors, which are notorious
for their "soft" transfer characteristic. GSM wouldn't be economically
possible without pre-distortion.

The transfer curve of the power amplifier is measured, and then one of
two things happens. If the solution is a hardware one, a matrix of
diodes and resistors is designed to straighten the overall curve.
Alternatively in software, the lookup table for the DAC that creates
the signal is changed to oppose the errors of the power amp.

d

--
Pearce Consulting
http://www.pearce.uk.com

Exactly the same procedure is used for high-power UHF Television
transmitters. Klystron distortion would be unacceptable without it, so Dif.
Phase and Dif. Gain are substantially reduced by pre-distorting the TV
signal. I remember one rather naughty incident when we had the Customer
doing Factory Acceptance, and we couldn't get both Dif Gain and Phase within
spec. at the same time, so the test engineers arranged for the Dif Gain
measurements to be made before lunch and Dif Phase after lunch. Whilst the
customer was having lunch, the Test Engineer changed the predistortion
characteristic so that both measurments would come out on spec. Not
surprisingly, the customer realised what was going on, red faces all round,
and sandwiches at the test bay rather than lunch thereafter.

S.



Naughty rather naughty;!.....


Weren't their some fussy sods from the BBC ever tangle with Sean
whatsisname from SCPD?...



--
Tony Sayer

RCA Dynagroove - Exactly what is it?
 

In article , Don Pearce
wrote:
On Sat, 19 May 2007 10:03:27 +0100, Jim Lesurf
wrote:

Your description would be more appropriate for a 'nulling' or
cancellation method where two sources are employed. But need not
involve any feeback. In the context of this thread, this distinction is
relevant as the 'dynagroove' system was a method which tried to cancel
or null one distortion with another - but this involved *no* feedback
mechanism to compare the two in reality. The LP cutting system had no
way to sense what the LP replay system would actually do when playing
the LP.


This is pre-distortion. It is currently widely used in RF power
amplifiers to lift the third order intercept point sufficiently to get
acceptable output power from RF power transistors, which are notorious
for their "soft" transfer characteristic. GSM wouldn't be economically
possible without pre-distortion.

The transfer curve of the power amplifier is measured, and then one of
two things happens. If the solution is a hardware one, a matrix of
diodes and resistors is designed to straighten the overall curve.
Alternatively in software, the lookup table for the DAC that creates the
signal is changed to oppose the errors of the power amp.

Yes, that is an example. However it has the advantage that the 'correction'
is applied on the basis of detailed knowledge of the behaviour of the item
which is to be corrected. In such cases no feedback need be present as it
relies on 'prior knowlege' which is reliable. The method is useful in cases
where feedback can be difficult to employ - e.g. in amplifiers where the
loop delay would be too large.

Nulling and pre-correction are perfectly sensible techniques in appropriate
circumstances, and can be used to avoid having feedback.

The problem for dynagroove was that the results were played on a wide
variety of systems whose behaviours differed. Thus no single 'correction'
would be optimum for all replay systems. I suspect the correction was also
crudely done, so probably wasn't optimum, but given the variety of replay
systems this was probably irrelevant in practice. Plus, I suspect, that
most people playing LPs didn't notice the distortion anyway - or liked it.
:-)

Slainte,

Jim

--
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