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View Full Version : MP3 quality poor due to low volume levels, not compression.



Simon Turner
2003-12-31, 03:46
I have just posted this to Hydrogen Audio. I am not sure about posting it
here as it is not really a Slimdevices problem. I have posted though in the
hope that there is some person here who has had the same problem and may
have got to the bottom of it. I am tearing my hair out. I am sorry the post
is very long but I was afraid I would get replies suggesting avenues I had
already explored otherwise.
---------------------------------

I encode my MP3 using the Hydrogen Audio recommended methods (EAC,
Lame, --alt-preset standard or extreme). After encoding I use MP3Gain to get
rid of any clipping that may have occurred (none usually). Due to the
problem described below I use it's Constant Gain function (per CD) to get
the MP3s as loud as possible (without introducing clipping). Using this
setting I still maintain the volume differences between tracks on any one
CD.

My MP3s sound fine when payed through a computer (with Terratech DMX 6Fire
soundcard and attached NAD hifi) or on the headphones of my iPod.

However when I attempt to use my Squeezebox (www.slimdevices.com) and my
iPod fed through their analogue line-outs to my main hifi the ouput volume
level is not high enough to drive the main amplifier correctly. So I have to
turn up the main amplifier volume control way above what i have to for my
Tuner, Video, CD player etc.
This results in a thin and weedy sound as the input amplifier of my main amp
is not being powered sufficiently by the output from the Squeezebox or the
iPod.

My amplifier is a Harmon Cardon.

It appears to me that when MP3Gain is used to get rid of the clipping the
volume of the MP3s are reduced to a lower volume than the Squeezebox and the
iPod are designed for.

I have another suspicion too, which seems a bit unlikely, but may be worth
throwing into the mix. It may be that my Harmon Cardon amplifier has some
sort of high input impedance and that this adversely affects the volume from
the Squeezebox and the iPod.. but the CD player and other hifi devices are
not affected by it (seems ulikely huh?).

One other thing, just to stop anyone who has an iPod from wasting their
time.
My iPod is a European model with the associated volume cap. I use euPod
Volume Boost to raise the volume. This works fine for the headphones but
does not affect the line out at all. However, I'm pretty sure that the
line-out is not capped, and anyway it is immaterial as both the Squeezebox
and the iPod suffer from the same problem, even though the guys at
Slimdevices say that the output of the Sueezebox is higher than that of most
other line-outs!

Any help or suggestions would be very appreciated as I have spent many hours
(or days, rather) on this problem and have got knowhere at all.
I am even considering upping the volume of all my MP3s even though this
would introduce clipping, as the clipping my not be as damaging to the sound
as the lack of volume!
I'd rather not use "normalisation" as I believe that this irreversably
changes the sound of the MP3s.

seanadams
2003-12-31, 10:20
On Dec 31, 2003, at 2:46 AM, Simon Turner wrote:

> I have just posted this to Hydrogen Audio. I am not sure about posting
> it
> here as it is not really a Slimdevices problem.

I can't answer this definitively but I might be able to offer a few
pointers...

> I encode my MP3 using the Hydrogen Audio recommended methods (EAC,
> Lame, --alt-preset standard or extreme).

Good.

> After encoding I use MP3Gain to get
> rid of any clipping that may have occurred (none usually).

I haven't used mp3gain... usually normalization (or I should say, lack
thereof) doesn't bother me too much. I think I just unconsciously reach
for the volume quite a bit. At any rate, one alternatively to look into
is whether you could normalize the audio before encoding. I don't know
if lame has an option for this, but you could do it with sox if you rip
to PCM first.

>
> My MP3s sound fine when payed through a computer (with Terratech DMX
> 6Fire
> soundcard and attached NAD hifi) or on the headphones of my iPod.
>

How about the squeezebox with headphones?

> However when I attempt to use my Squeezebox (www.slimdevices.com) and
> my
> iPod fed through their analogue line-outs to my main hifi the ouput
> volume
> level is not high enough to drive the main amplifier correctly. So I
> have to
> turn up the main amplifier volume control way above what i have to for
> my
> Tuner, Video, CD player etc.

That's odd, and it may indicate that your MP3s aren't normalized to
100%.

> This results in a thin and weedy sound as the input amplifier of my
> main amp
> is not being powered sufficiently by the output from the Squeezebox or
> the
> iPod.
>

The only kind of problem you should really expect to hear from a
too-low signal source is more noise.... you have a lower signal for the
same noise level at the DAC (and cables), therefore a lower s/n ratio.
Also if it is due to incorrect normalization, you also get more noise
in the digital path because of the reduced dynamic range. I don't know
what you mean by "thin and weedy" but I guess you mean something other
than just plain "noisy". I'm thinking it might be worth trying another
receiver, or even (carefully!) connecting the squeezebox/ipod directly
to your main amp and seeing how it sounds.

> It appears to me that when MP3Gain is used to get rid of the clipping
> the
> volume of the MP3s are reduced to a lower volume than the Squeezebox
> and the
> iPod are designed for.

Could be...

> I have another suspicion too, which seems a bit unlikely, but may be
> worth
> throwing into the mix. It may be that my Harmon Cardon amplifier has
> some
> sort of high input impedance and that this adversely affects the
> volume from
> the Squeezebox and the iPod.. but the CD player and other hifi devices
> are
> not affected by it (seems ulikely huh?).

By high, I think you mean low. Not sure about the ipod, but squeezebox
will drive anything down to 16 ohms (i.e. headphones) on either the RCA
or 1/8" outputs, so I'm sure this is not the problem.

>
> One other thing, just to stop anyone who has an iPod from wasting their
> time.
> My iPod is a European model with the associated volume cap. I use euPod
> Volume Boost to raise the volume. This works fine for the headphones
> but
> does not affect the line out at all. However, I'm pretty sure that the
> line-out is not capped, and anyway it is immaterial as both the
> Squeezebox
> and the iPod suffer from the same problem, even though the guys at
> Slimdevices say that the output of the Sueezebox is higher than that
> of most
> other line-outs!
>

Actually it is possible for us to get even higher levels on the analog
outputs, but it's a compromise because it means you have to reduce the
digital gain a bit to get the maximum levels, and this makes things
noisier at the much lower levels usually used with headphones.
Squeezebox has both digital and analog volume control capability. By
default, we use digital volume control with fixed analog gains, because
everybody expects the volume control to work for the digital outputs.
However, for those who want the best quality possible from the analog
outputs, we support a fixed digital volume control with variable analog
gain. This is a new feature and it does need some further tuning, but
you can try it out by going to Player Settings -> Additional Settings.
I'm planning to expose more of the audio controls for people who really
want to tweak it for a particular setup (eg for driving an amp
directly, you'd want to use only the analog gain, and possibly adjust
the available range).

> Any help or suggestions would be very appreciated as I have spent many
> hours
> (or days, rather) on this problem and have got knowhere at all.
> I am even considering upping the volume of all my MP3s even though this
> would introduce clipping, as the clipping my not be as damaging to the
> sound
> as the lack of volume!
> I'd rather not use "normalisation" as I believe that this irreversably
> changes the sound of the MP3s.

Well, the last thing I would suggest is of course to try the digital
output so you can compare with another DAC.

Ron Thigpen
2003-12-31, 14:07
Sean Adams wrote:

> On Dec 31, 2003, at 2:46 AM, Simon Turner wrote:

>> After encoding I use MP3Gain to get rid of any clipping that may
>> have occurred (none usually).


> At any rate, one alternatively to look into is whether you could
> normalize the audio before encoding. I don't know if lame has an
> option for this, but you could do it with sox if you rip to PCM
> first.


EAC does have built-in normalization that works at the WAV level. You
might try this as an alternative to the MP3 level normalization. I don't
recall if the EAC normalization is done at the track or album level.

--rt