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Subject: Re: Recorder noise
From: Andrew Jamieson (ajamiesn_at_mira.net)
Date: 2002-02-15


Hmmmm. Now I don't have a recorder (although donations will be kindly
accepted ;) so I'll have to work off the schematics. A few things jump out
immediately:

* If the noise goes away when the music is stopped, then it's not the amp
that's causing it, but the pre-amp. This is kinda moot, seeing as the two
parts are integrated into the MAS chip, but it could be helpful later on.

* The recorder has _very_ little by pass and filter capacitance when
compared to the recommended implemetation in the MAS datasheet. The
datasheet shows three filter caps on the main power supply, ranging from pF
to uF, to filter out different frequencies on the power rail. If the noise
is of a very low frequency (such as 15hz) then the Archos may be missing the
higher rated caps (uF).

* The schematic does not show a capacitor to PVDD. This is an auxilary
supply to the analog circuitry, and is specified as requiring a 3nF bypass
cap to AVDD1. It's possible that hooking this up would help ....

* The input lines on serial audio input (SIC, SID, SII) are left
floating. It is possible that these are picking up noise from the circuit
and causing problems. I would recommend tying these high (I think ... the
datasheet is a little unclear on this, which is probably why they are left
floating. Best to tie them to something tho ...)

My best bet at the moment is the PVDD cap, and maybe some additonal bypass
caps (but modding these in a completed circuit is often counter-productive).

Hope this helps ......

A

PS: I'll take a look at some of the pictures of the recorder on the net,
and see if I can see anything else ....

----- Original Message -----
From: "Björn Stenberg" <bjorn_at_haxx.se>
To: "Rockbox development" <rockbox_at_cool.haxx.se>
Sent: Saturday, February 16, 2002 12:23 AM
Subject: Recorder noise

I recently got myself a second Archos, a Recorder 20. I figured I should
have both a player and a recorder if we're going to write code for both.

The Recorder is really nice, with much more powerful audio. Unfortunately,
it also has a very faint, but noticeable, noise in the audio output. At
first I didn't think it would bother me, since it isn't noticable while
the music is "normally loud". But I listen movie scores a lot and they
often have long sections of very still and quiet music, in which the noise
is very annoying.

I figured with all the electronics wizards in here, we could perhaps try
and analyze what the noise comes from and if there's something that can be
done about it.

The noise is most apparent during playback (including in paused mode),
possibly due to the VU bars that are updated on the screen. It then has a
oscillating sound at a frequency of maybe 10-15 Hz.

If I bring up the menu or another screen without the VU bars (during
playback), the oscillating stops and there is a instead a steady white
noise.

Stopping playback stops the noise. Also, the noise goes away during disk
access.

Ideas, anyone?

--
Björn



Page was last modified "Jan 10 2012" The Rockbox Crew
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