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A7N8X-DLX Mic Input Noise Level Inquiry

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Anonymous
a b V Motherboard
September 14, 2004 8:26:33 PM

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

I have a nifty WAV editor program that came with Nero that indicates
the signal-noise ration (S/N) on the Mic input as being pretty poor
with the Mic boost enabled. I also see the Granted, the Mic boost is
merely amplifying the input, but with Mic boost enabled, I see only
about 35 dB S/N on the Mic input with or w/o a Mic plugged in. I see
about -35 dB on the recording display with the Nero wav editor with
the "record" mode active. Anyone see similar readings?

When I disable the Mic boost, the noise level drops, yet one channel
still has about 8 dB more noise than the other. This symptom appears
to be identical with both of the A7N8X-DLX mobos that I own so I don't
think it's PS noise. The box that has the more expensive PS actually
has more noise on the Mic input w/o any Mic plugged in. Just some
observations to ponder and discuss.

--
Best regards,
Kyle
September 15, 2004 1:29:00 PM

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

In article <2qqargFucg0eU1@uni-berlin.de>, "Kylesb" <me@privacy.net> wrote:

<<snip>>
> I should have mentioned I did my tests with all inputs disabled except
> for a single recording source. This appears to be the way the mixer
> software is designed to function, only 1 recording source can be
> enabled at a time (I recall other sound cards, particularly SB16
> cards, would allow recording from multiple sources, I think).
> Enabling "digitize input" subjects the input signal to the processing
> of the digital audio circuits/drivers. Enabling "digitize input" may
> make the noise better or worse depending upon your equalizer and
> "environment" settings.
>
> I can't recall if I have the 4-wire CD audio cables installed in this
> box.
>
> The noise floor is not awful, on the contrary, it's decent or lower
> than -50 dB which gets the noise floor down where one will not hear it
> above typical computer fan noise or room noise. But, the mic boost
> clearly raises the noise floor 20 dB, which is to be expected since
> it's simply an amplification of the mic input signal.
>
> I did notice tonight while playing with the 3com NIC (which has been
> disabled for some time, but I'm using it for a brief span for some
> testing purposes) that a network speed test using Sisoft Sandra
> resulted in identifiable noise in the headphones I was using during
> the brief burst of data, and I never noticed any such noise with the
> Nvidia NIC. The Nforce thread you linked above where users suggest
> some cures is humorous in a way, b/c each users seemed to find the
> magic fix to noise, e.g., paper washers, using only the 3com nic,
> using only the Nvidia NIC, disabling a PCI bus service,
> enabling/disabling "digitize input", etc.
>
> It is clear the multifunction LSI circuits that include onboard sound
> face huge design hurdles in establishing a low noise level. In
> addition, the location of the audio signals in the mobo (near the CPU)
> puts those signals in a very noise environment. However, I thought to
> get and use the Dolby Digital trademarks, certain performance levels
> were required in the S/N area, or maybe I am mistaken.
>
> I'll have to have a look at my system with the SBLive 5.1 and see if
> it's noise floor is any better, just a sec. . . . (/me fiddles with
> sblive 5.1 card in an older K7S5A based 1.4G Tbird system)
>
> No doubt about it, the SBLive card is much better in the noise floor
> area, the mic input had lower than -70 dB reading with no mic plugged
> in and "mic boost" enabled, and "line in" noise floor was less
> than -60 dB level (I do have the "line in" jack connected to a stereo
> receiver at the moment as I'm recording old LPs and making MP3s
> therefrom, so that might explain the difference in noise floor there).
> One observation I note, the SBLive board has a good quantity of very
> small electrolytic caps, typically used for noise filtering in audio
> circuits.

But this is not your typical "noise filtering" problem.

First of all, the AC97 codec does the digitizing. Transmission from the
Realtek chip (ALC650?) on the left of the board to the Southbridge, is
done digitally over the AClink.

The noise problem will exist from the audio connector rack in the I/O
area, to the AC97 codec. Now, I looked at two A7N8X family boards, and
I can see audio signals travelling on the surface of the board. This makes
them "antennas" for noise, and is a definite no-no. They should be routed
stripline, with solid ground plane over top of them, to protect against
E fields. Also, there should be sufficient clearance between them and any
H fields, like the transformers for the Ethernet interface.

The whole Asus nforce2 layout is compromised by the dual channel memory
interface. It pushes the processor socket and Northbridge too far to the
left, and forces other circuits to be placed sub-optimally. If you want
to see a better layout, compare an A7N8X board to the Abit NF7-S (the
first one I grabbed).

http://www.abit-usa.com/images/products/NF7_S_large.jpg

Notice how the DIMMs are much closer to the front of the computer,
the Vcore power circuit is pushed North, away from critical routing
areas near the I/O panel. The area south of PCI5 has been used for
chips/headers. The layout is less crowded where the audio chip lives,
and there is more opportunity to treat the audio right. (I wish I had
a hi res picture, to be able to see more of it).

If you remember the early history of the A7N8X, Asus did lose their
SoundStorm "sticker", and who is to say why. It could be those very
things, like measured noise floor and quality of implementation.

Imagine how good a board like this could have been, if an extra two
routing layers were available. Adding 2 more layers of copper adds
10% to the cost of the PCB board blank, but it would have allowed
that huge copper "rats nest" between the Northbridge and the DIMMs
to be compressed, and allowed the use of ground fill around circuits
that really needed it.

Also, the audio circuit is powered by a small linear regulator. In
theory, this is supposed to clean up power to the circuit, but I've
been taught that in fact a switching power supply can be cleaner
than a linear regulator. The linear regulator rejects noise well at
low frequencies, but not at high frequencies, and the +12V used to
power the regulator, is bound to be dirty after travelling half way
around the board, to get to the audio circuit. Yet, I don't see any
RLC filters feeding the regulator, to improve its output.

So, there are a bunch of things that could be done differently. As the
board is intended for budget AMD applications though, it is hard to
do the job right. The NF7-S layout proves there is room for
improvement, in terms of general layout issues.

A sound card, on the other hand, has no compromises in its design.
Plenty of room on a PCI card, to do an audio function, without
messing it up.

Paul
Anonymous
a b V Motherboard
September 15, 2004 3:44:55 PM

Archived from groups: alt.comp.periphs.mainboard.asus (More info?)

"Paul" <nospam@needed.com> wrote in message
news:nospam-1509040929440001@192.168.1.177...
| In article <2qqargFucg0eU1@uni-berlin.de>, "Kylesb" <me@privacy.net>
wrote:
<snip>
|
| But this is not your typical "noise filtering" problem.
|

I was not very specific in my capacitors/noise filtering comments, but
plenty of caps are typically used to filter the rail voltages in good
audio circuits, I was not intending to suggest in-line "audio
filtering" circuits. That was the intent of my comment about the
electrolytics. Yes, I understand the audio inputs are supplied to a
codec that xforms audio into digital data.

| First of all, the AC97 codec does the digitizing. Transmission from
the
| Realtek chip (ALC650?) on the left of the board to the Southbridge,
is
| done digitally over the AClink.

I agree entirely with your observations, if you place audio circuits
into a digitally noisy environment, special consideration must be
given to noise filtering, the placement of PCB runners and shielding.

| The noise problem will exist from the audio connector rack in the
I/O
| area, to the AC97 codec. Now, I looked at two A7N8X family boards,
and
| I can see audio signals travelling on the surface of the board. This
makes
| them "antennas" for noise, and is a definite no-no. They should be
routed
| stripline, with solid ground plane over top of them, to protect
against
| E fields. Also, there should be sufficient clearance between them
and any
| H fields, like the transformers for the Ethernet interface.
|
| The whole Asus nforce2 layout is compromised by the dual channel
memory
| interface. It pushes the processor socket and Northbridge too far to
the
| left, and forces other circuits to be placed sub-optimally. If you
want
| to see a better layout, compare an A7N8X board to the Abit NF7-S
(the
| first one I grabbed).
|
| http://www.abit-usa.com/images/products/NF7_S_large.jpg
|
| Notice how the DIMMs are much closer to the front of the computer,
| the Vcore power circuit is pushed North, away from critical routing
| areas near the I/O panel. The area south of PCI5 has been used for
| chips/headers. The layout is less crowded where the audio chip
lives,
| and there is more opportunity to treat the audio right. (I wish I
had
| a hi res picture, to be able to see more of it).
|
| If you remember the early history of the A7N8X, Asus did lose their
| SoundStorm "sticker", and who is to say why. It could be those very
| things, like measured noise floor and quality of implementation.

As a relative latecomer to the A7N8X product line, I was unaware Asus
lost their "sticker", yet it's not hard to understand why given the
results of a few simple S/N tests.

|
| Imagine how good a board like this could have been, if an extra two
| routing layers were available. Adding 2 more layers of copper adds
| 10% to the cost of the PCB board blank, but it would have allowed
| that huge copper "rats nest" between the Northbridge and the DIMMs
| to be compressed, and allowed the use of ground fill around circuits
| that really needed it.

Adding $10 to the cost of a mobo could sound the death knell w/r/t the
sales volume thereof. However, if the company paid attention to
marketing the board properly, an added $10 might be easy to extract
from consumers if the board delivered superior performance in the
on-board audio arena. Asian rim companies often times are clueless on
how to best market their products to the US consumer, imho. If it's a
DLX version, then by golly, it oughta get all the bells and whistles
and superior design features.

|
| Also, the audio circuit is powered by a small linear regulator. In
| theory, this is supposed to clean up power to the circuit, but I've
| been taught that in fact a switching power supply can be cleaner
| than a linear regulator. The linear regulator rejects noise well at
| low frequencies, but not at high frequencies, and the +12V used to
| power the regulator, is bound to be dirty after travelling half way
| around the board, to get to the audio circuit. Yet, I don't see any
| RLC filters feeding the regulator, to improve its output.

Filtering the output of the linear ps is my preferred approach,
specifically 1 uF filter caps at every IC power connection pin to help
reduce high frequency noise. When designing audio circuits in years
gone by, I much preferred using linear supplies as no matter what the
situation, switching circuits will cause noise in the voltage rails.
In fact, a design I worked on years ago with microprocessor controls
and lots of audio circuits used a quality linear PS for all the audio
circuitry and a separate switching supply for the microprocessor
circuits to achieve 96 dB S/N ratios throughout. Heh, it was amazing
what we could do at that time with a 2-3 Mhz clock speed CPU, lol (an
8085 CPU was used to drive a VF dot matrix display, it was a 4 channel
audio analyzer used in audio output testing of radio and cassette tape
products, it even performed wow and flutter tests on tape decks).

|
| So, there are a bunch of things that could be done differently. As
the
| board is intended for budget AMD applications though, it is hard to
| do the job right. The NF7-S layout proves there is room for
| improvement, in terms of general layout issues.

I guess the DLX board is still considered budget, even with the higher
price. Guess I'm getting spoiled with the price of things these days.

|
| A sound card, on the other hand, has no compromises in its design.
| Plenty of room on a PCI card, to do an audio function, without
| messing it up.

As always, your comments are worthy of note Paul.

--
Best regards,
Kyle
!