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Subject: Re: Getting rid of that damn noise.
From: Rob Ward (wards_at_paradise.net.nz)
Date: 2001-12-27


My understanding of LCD displays is that they use a low frequency clock to
drive them, and a voltage converter to generate a negative (-5V?) voltage.
Reading your email, I see the problem disappears when the HDD is accessed,
however the clock driving the LCD must still be toggling at this point if
you can read the display. (Unfortunately I don't have a schematic in front
of me).

Voltage converters/inverters are by nature damn noisy things. (Just
thinking...Inverters normally operate at say 30kHz which is way above
audible hum, so I kind-of doubt the problem is directly from the inverter).

My first suspicion would be noise on the power rails is causing the problem,
which subsequently disappears when the low impedence motor for the HDD is
switched into the circuit, absorbing some power supply rail ripple.

I suggest looking at the power rails with a scope (AC coupled) and look for
some mV (millivolts) of noise. This may be being picked up and amplified by
the output amplifier, or maybe the rail noise is just not being rejected by
the amplifier in which case a wee bit of RC filtering at the op-amp supply
may fix it.

If it is easy to fix, I wonder why the manufacturer did not do it, or maybe
you have a early version?

By the way, did you buy your unit in NZ? I am looking at importing one from
the US.

Rob Ward (Wellington - New Zealand)

----- Original Message -----
From: "Justin Fisher" <jaf60_at_it.canterbury.ac.nz>
To: "Jukebox" <jukebox_at_cool.haxx.se>
Sent: Thursday, December 27, 2001 9:01 PM
Subject: Getting rid of that damn noise.

> I'm just skimming though some of the incredible work you guys have done
> (the entire archos circuitry mapped out - amazing! Even if I can't
> follow most of it :-), and my thoughts return to finding a way to get
> rid of that annoying background hum in the AJBR (the one that ceases
> whenever the JBR accesses the HDD, thus we know it is possible for the
> JBR to play mp3's without the noise).
>
> Seeing as there are only four lines coming out of the CPU to operate the
> LCD (the LCD system seems to be the culprit as the hum frequency matches
> the screen updates, goes silent when the screen freezes (ie during HDD
> reads) yet the CPU is presumably active during this silence), I was
> wondering if anyone wants to, er... cut them. :-)
>
> I figure that adding a switch to your unit that simply switches off the
> LCD section, and thus switches off the background hum, would be a great
> modification. And pretty easy to do. UI-wise, it is virtually no effort
> to flick the LCD back on if you want to check something, and when you're
> listening, you're not looking at the LCD screen anyway. Awesome sound
> quality at the flick of a switch. That is assuming my layman deductions
> hold water and the noise is indeed eminating somewhere between the LCD
> screen and the CPU.
>
> Does it sound like something worth investigating? If removing the noise
> was as easy as installing a switch on the CPU outputs that go to the
> LCD, I'd be overjoyed.
> If it is that easy, would there be complications? (ie components not
> liking being hot-plugged like that, or several other lines also needing
> to be switched? Would simply cutting the power to the LCD driver (a mere
> one line) and leaving the CPU data lines connected suffice?
>
> Thoughts?
> Barking up the wrong tree?
>

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