Re: Quad FM4 Bass : was Re: FM recording level?
- From: Jim Lesurf <jcgl@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 05 Jul 2005 15:57:52 +0100
In article <slrndcis9b.5uh.news0309@xxxxxxxxxxxxxxxxxx>, John Phillips
<news0309@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> On 2005-07-04, Jim Lesurf <jcgl@xxxxxxxxxxxxxxxxxx> wrote:
> The faint 18 kHz feature is:
> - on both left and right chanels;
[snip]
> I think it's on the incoming signal. Maybe it's a residual from the RDS
> signal. However the RDS carrier is at 57 kHz in the transmitted signal
> I think. I think that would put a residual, via the L+R/L-R matrix, at
> 19 kHz. Maybe what I see is the residual after the MPX flter takes some
> of it out - maybe the peak I see at 18 kHz is part of a sideband rather
> than a carrier.
[snip]
> I was speculating about the FM4's design, that if the bandwidth of the
> L-R subcarrier demodulator is rather wide [1], this might allow the RDS
> sub-carrier and sidebands to enter (and exit) the L+R/L-R -> L/R matrix.
> If the subsequent 19 kHz filter is narrow then I suspect some leakage of
> the RDS subcarrier's sidebands might be possible.
> I can't think of another mechanism right now to allow low-level traces
> of signal through centred on 18 kHz.
I'm also not sure where 18 kHz may be coming from. Afraid I don't know
enough about RDS, but am wondering if it is a nonlinear product of the RDS
and the stereo subcarrier. The clock rate of the RDS may perhaps create
sidebands of 57kHz that then mix with the 38 KHz to give a result around 18
kHz. But I can only guess at that.
FWIW The FM4 uses a KB4441 IC as the final part of the IF and as an FM
demodulator. This passes its output to the stereo decoder IC (TCA4500A) via
an RLC low-pass filter. 10k pot and 27 microH in series, then an 180pF to
ground. So far as I can see, though, the pot adjustment (RV5) is only
listed under optimising the crosstalk. No info on the typical response.
Can't see any other 'anti birdie' filtering between the demodulator and the
decoder.
> Of course my FM4 (vintage 1983 and probably in need of a service) could
> be actually faulty. Regrettably I have no other analogue tuner to try.
> [1] On the premise of leaving a deep and fairly narrow 19 kHz filter to
> remove the 19 kHz pilot tone (there being no modulated RDS carrier
> at 57 kHz to remove when the FM4 was designed).
Alas, the service manual doesn't give a toko part number that I immediately
recognise. I'd expect the filter to start a roll-off at about 15kHz, have a
dip/null at 19kHz, and then a fall away. I have plots for some filters, but
don't know if the quad ones would be identical to any of them.
Slainte,
Jim
--
Electronics http://www.st-and.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html
.
- References:
- FM recording level?
- From: John Phillips
- Re: FM recording level?
- From: Jim Lesurf
- Re: FM recording level?
- From: John Phillips
- Re: FM recording level?
- From: tony sayer
- Quad FM4 Bass : was Re: FM recording level?
- From: Jim Lesurf
- Re: Quad FM4 Bass : was Re: FM recording level?
- From: John Phillips
- Re: Quad FM4 Bass : was Re: FM recording level?
- From: Jim Lesurf
- Re: Quad FM4 Bass : was Re: FM recording level?
- From: John Phillips
- Re: Quad FM4 Bass : was Re: FM recording level?
- From: Jim Lesurf
- Re: Quad FM4 Bass : was Re: FM recording level?
- From: John Phillips
- FM recording level?
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