[s-cars] Speaking of Ipods NAC (long)

[Dave Ellis]

Hi Dave!

Last summer I spent a bit of time evaluating various MP3 players for 
sound quality and features (my wife wanted one).  I'm a bit of an 
audiophile, so tend to be picky about such things.  In the end the iPod 
won out over others for both features and sound fidelity, although it 
also cost more than most of the other contenders too.

djdawson2 at aol.com wrote:
> So... my questions:
> 1) Is there a high quality interface available for car stereos?

Yes, many new head units come with built in iPod interfaces.  The 
ability to navigate the iPod's menus and playlists does vary in quality.

If your head unit doesn't have an iPod interface, you can buy interface 
adapters for most head units (including the '98 and newer Audi head 
units).  Try to avoid ones that use the headphone jack on the iPod, and 
instead look for ones that use the iPod's docking connector on the 
bottom of the iPod.  Not only does this ensure the best sound quality by 
using the line level outputs rather than the amplified headphone output, 
but in most cases will also allow you to navigate the iPod's music 
playlists using the head unit's CD controls.  As an added bonus, most 
will also charge the iPod and provide power as you drive rather than 
relying on the iPod's batteries.

> 2) I don't see hi-fi type specs related to Ipods.  What is the sound quality?

Sound quality of the last few generation iPods is quite good.  The 
latest 5th generation iPods sound quality is slightly improved over 
previous generations (and as an added bonus you get video playback as 
well).

That said, the output sound quality is largely dependent upon your 
source music file.  The iPod can play back audio files in a few 
different formats.  MP3 formated files (the most widespread) and Apple's 
proprietary AAC formated audio files both use lossy compression to 
reduce the file size to allow more music files to fit into the available 
storage.  Since the compression is lossy though, this means that there 
will always be some level of sound degradation when using a compressed 
file format, but if done well enough it shouldn't be too noticeable or 
not noticeable at all.

MP3 files vary in quality and are most typically separated by talking 
about the data rate they are generated for.  The higher the data rate 
the less the file is compressed and the better the sound quality, but 
the more space the file will take up.  Typical data rates for MP3 files 
are 112kbps (kilobits per second) to 320kbps for music.  For cheap 
headphones and speakers, 128kbps is generally adequate, but if you are 
playing back through quality equipment then you will benefit from using 
at least 192 to 224 kbps as the minimum to keep the sound close to CD 
quality.

Alternatively, you can transfer the music file in it's original format 
uncompressed from the CD to your iPod in a WAV file format.  There is no 
compression, so no loss in sound quality and the music output will sound 
exactly as it does from the CD.  The downside however is that the WAV 
files will be so big that you won't be able to fit anywhere near as many 
on your iPod as you could MP3 files.

Now in reality, the noisy environment inside a car (even with good 
quality sound gear) should render the differences between a 192kbps MP3 
file and an uncompressed WAV file indistinguishable.  Even 128kbps may 
be good enough.  I haven't been able tell the difference between an MP3 
and original uncompressed WAV file in any car I've listened to so far, 
even when parked.  So I wouldn't sweat it too much for listening on the 
road.

If you're transferring your music collection to MP3 format for listening 
in a variety of locations however, I'd tend to go with higher bit rates. 
  Using my wife's iPod on our home audio system or when playing MP3s 
back via the attached PC, there is clearly an audible difference in the 
sound quality between 128kbps and 192kbps files.  Going to 224kbps is a 
little more subtle, and original CD's sound slightly subtler still.

The biggest difference between MP3 files recorded at 192 or 224 kbps and 
the uncompressed music from the original CD is primarily found in the 
soundstaging and openness of the music (MP3 files even at higher bit 
rates reduce the soundstage of most recordings quite a bit), and in the 
background details.  For example, in the uncompressed music from the CD 
you can often hear the sound of an indrawn breath or fingertip brushing 
the string of an acoustic guitar in a good recording, but once 
compressed to an MP3 file those details tend to be lost or dramatically 
muted or distorted.  For some, this probably won't matter much, but 
then, as I said in the beginning of this email, I tend to be picky about 
my audio.  :o)

> 3) Is the signal coming from an Ipod equivalent to the signal quality from a nice CD player?

It all depends what you mean by a "nice" CD player.  :o)

The sound is as good as (or maybe even a little better than) most 
mass-market CD players found in most of the big box stores these days. 
If you want something a little nicer there are a few places that will 
modify your iPod for a few hundred dollars, replacing capacitors, 
resisters, etc, bypassing the cheap amplification stage, adding better 
quality DACs, and what not.  But really, for most conceivable situations 
it's just fine as is.  If someone wanted serious fidelity for listening 
they probably wouldn't be using an iPod to play back compressed music in 
the first place.

> 4) Wide frequency response?

20Hz-20kHz is the listed output.

> 5) 90+ db signal to noise ratio?

I remember finding audio specs in some review for the 5th generation 
video iPod, and though I can't find it right now if I recall correctly 
S/N was something like 102db and THD was also reasonable and comparable 
to most general consumer grade CD players.  These specs were tested for 
from the line out found on the iPod connector (found at the bottom of 
the iPod).  Specs for the headphone jack were slightly worse, as can be 
expected from the so-so amplification stage used for the headphone output.

> 6) Does anyone make a head unit that effectively serves as an Ipod controller?

Yep.  Most manufacturers offer iPod connectivity on at least some of 
their models now.

> Anyway... any enlightenment would help.  Sure seems like it would be nice to replace skipping and large quantities of CDs with a box the size of a small calculator.
> Thanks,
> Dave

You're welcome!  This reply got a little longer than I had originally 
intended, but Audio for me is right up there with nice cars on my 
interest-o-meter.

Hope it helps, and sorry if I got too detailed in some places.
-Dave