/usr/share/doc/rakarrack/html/hardware.html

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<h2>Hardware Connections</h2>

<p>It is not an uncommon situation where the only audio input interface to 
a computer is a microphone or a line input on a standard consumer grade 
sound card.  While the modern day consumer grade sound cards actually 
have admirable performance specifications, the interfaces are designed 
for certain types of inputs.  If the device plugged into the sound card 
is not designed to work with the interface, then you will get results 
that are characterized by noise, poor frequency response, or 
distortion.  Most people will complain about how Rakarrack is noisy, or 
the guitar sounds bad.  We have no software solution to defy the laws of 
physics.</p>

<p>However, we do have some recommendations to improve your hardware 
connections to minimize the problems associated with your sound card.  
Line in and Mic inputs will be discussed separately since they are 
somewhat different in the ways they interact with the input devices.</p>

<p>The shortened, non-technical advice about Line and Mic inputs is you can 
reduce noise significantly by plugging a device designed to receive the 
instrument and drive the Line or Mic input according to the way it is 
designed.  For Line in, a preamp, a guitar with active electronics will 
work, a keyboard with a line out, or something similar are devices that 
act like the type of device the sound card expects.</p>

<p>Something with a headphones out is probably a bad idea but sometimes 
worth a try.  Often a headphone output uses a small switching 
amplifier.  The high frequency switching noise is filtered by your 
headphones, but may cause noise or a whine in your sound card. Headphone 
outputs that may work are those using normal linear amplifiers and have 
low output impedance.</p>

<p>Here is some more worthwhile reading about the topic:</p>

<p><a href="http://www.musiconmypc.co.uk/art_record_guitar_on_pc.php">http://www.musiconmypc.co.uk/art_record_guitar_on_pc.php</a></p>

<p><b>Line Input</b>:</p>
<p>A line input presents two problems for a mono instrument with high 
output impedance (typical electric guitar).  The first problem is the 
mono-to-stereo conversion.  This can be easily overcome with an adapter 
that either shorts one of the unused channels (something like a 
Y-splitter), or feeds the mono signal equally to both stereo inputs.  A 
splitter may be desirable because you can still use the unused channel 
for some other input such as a microphone.  This is not a difficult 
challenge because most adapters will short the unused input by default.  
Avoid cheap adapters that don't default ground unused inputs.</p>

<p>The most notable problem presented by a Line Input is impedance 
matching.  A typical Line Input has an input impedance of 10k-ohms.  A 
guitar pickup typically has 5k-Ohms impedance, but with the volume pot, 
this can be up to 250k to 500k series resistance, depending on the value 
of the pot.  This means with the volume pot wide open, the guitar signal 
received at the sound card will be 1/3 as loud.  Move the volume pot 
only a small amount, and the sound will be almost inaudible.</p>

<p>Not mentioning the noise and poor frequency response problems, this 
presents an Analog/Digital conversion problem.  In cheap sound cards, 
there is no analog programmable gain amplifier, so you have to do 
software amplification.  Not using the audio card's full scale 
resolution is tantamount to having a soundcard with lower resolution.</p>

<p>Take a 16-bit sound card, for example.  First, the guitar only puts out 
1V max peak for hot pickups, but the average guitar pickup gives perhaps 
1/5 of that most of the time.  Take 16 bits = 96dB dynamic range.  Take 
away 14 dB (~1/5) for guitar signal raw power compared to expected line 
level input, then take away 9.54dB (~1/3) for loss on input impedance, 
and now you have a dynamic range of 72dB.  This means you have reduced 
your 16-bit sound card down to a 12-bit sound card simply by not using 
the full input signal range.</p>

<p>If you back your volume knob to 1/2 way, this creates a series 
resistance of about 250k, and the division on the input is even worse.  
Backing the volume knob to 1/2 reduces your sound card to the equivalent 
of an 8-bit sound card, which is the grade of electronics you would find 
in a cheap toy.</p>

<p>Higher quality sound cards have a programmable gain analog amplifier to 
boost the signal before it goes to the A/D converter.  In this case, the 
built-in amplifier boosts the signal up where it can use the full 
resolution, but it also amplifies the noise on the line by the same 
amount.  In this case the signal/noise ratio is reduced by ~22dB (using 
example above), and you still have a non-optimal condition.  (As a side 
note, this is the same problem when plugging a guitar directly into a 
mixer board without a preamp--fortunately pro-quality mixer boards 
typically have less input amplification noise so you can often get away 
with using a passive direct box).</p>

<p>Not only do you have problems as indicated above, but the low impedance 
loading on a guitar pickup is a problem.  Most guitar pickups have a 
poor frequency response when loaded to less than 47k-Ohms impedance.  
Even worse, because of the electro-mechanical properties governing how 
guitar pickups work, a low impedance load performs the equivalent of an 
electromechanical brake, similar to what train engines use to stop the 
train.  The sound card input burns off the energy in your guitar strings 
in the form of heat (not dangerous for the sound card since this is low 
level energy).  More plainly, your sound card input is causing the 
guitar pickup to act as a magnetic brake on the guitar strings.  You 
will play a chord and it will appear to damp more quickly.  The singing 
sustain you get from your guitar amp is gone--not because the guitar amp 
is magical--but because the low impedance sound card electrical load is 
burning the energy out of your guitar strings while the guitar amp input 
impedance is designed to work well with most guitar pickups.</p>

<p><b>The solution?</b></p> 

<p>Here are a few:</p>
<p>Get or make a preamplifier that has a high input impedance for your 
guitar and a low impedance line-driver output.  Sometimes these are 
marketed as "line driver" or "active direct box".</p>

<p>Purchase a sound card with inputs designed for your instrument or 
microphone type.  Behringer seems to be the low cost solution.  They 
have such an audio interface for something near $40USD intended for use 
with guitar:</p>

<p><a
href="http://www.behringer.com/EN/Products/UCG102.aspx">http://www.behringer.com/EN/Products/UCG102.aspx</a></p>

<p>These are well supported under Linux.  If that cost is more than you can 
spare, and you don't happen to have any other kind of preamp device 
laying around, then you will simply have to plug direct to the line 
input or mic input and understand it could be better.</p>

<p></b>Microphone Input</b>:</p>
<p>Many of the same issues apply to this interface as to Line In, but there 
is one more caveat with the Mic inputs.  A typical PC microphone is a 
device with an active amplifier built-in, and this device expects to be 
powered by the computer.  The microphone input from a PC provides a bias 
voltage, and it is designed to expect a certain electrical load to be 
applied.  Most of the time the guitar loads this input enough to work, 
but in some cheap sound cards, you get DC offset (which can wreck some 
of your presets).   In all cases, the mic input is generally lower 
quality and higher noise, no matter what you do.  Here is a page with 
some good ideas for somebody with willingness to try a hand at simple 
electronics:</p>

<p>http://www.epanorama.net/circuits/line_to_mic.html</p>

<p>Something similar could be achieved with a 50k volume pot turned most of 
the way down so the one end has a low ~100-ohm impedance to ground, and 
the guitar gets most of the 50k in series.</p>

<p>The pickup loading and string braking as explained for Line Input is 
generally not a problem because you usually have to turn the volume knob 
back to attenuate the signal level to the low level expected by the mic 
input.  The resistance of the volume pot in this case is put in series 
with the pickup and thus loads the guitar adequately.</p>

<p>Regarding DC-offset:  DC offset is a constant value other than zero when 
the instrument is silent.  This will mess up some Rakarrack presets if 
left active on the line going into Rakarrack.  We have provided a 
solution to this problem.  In Settings->Preferences->Audio, put a check 
next to Filter Input DC Offset, and Rakarrack will take this out of the 
signal before doing normal processing.  It is an option you may 
enable/disable as needed because it is a waste of CPU usage if you have 
a good audio input that doesn't put DC into the system, and also is 
somewhat bandlimiting (the cut-off is -3dB at 20Hz, but above the 
cut-off a filter has some effect -- up to -3dB.  Above 50Hz, the 
band-limiting effect is negligible for audio purposes).</p>

<p><b>The Conclusion</b></p>
<p>The hardware configurations are listed from best to worse below:</p>
<p>1) Buy a high quality hardware digital audio interface that is designed 
for your instrument.</p>
<p>2) Use Line Input with a good quality preamp that is designed for your 
instrument. Boost the input level to use full resolution (VU bars on 
Rakarrack in the yellow).</p>
<p>3) Use Microphone input with some impedance matching circuitry to reduce 
noise and tame signal levels.</p>
<p>4) Use Microphone input level direct with guitar and adjust volume knob 
to get the optimal sound.  Use DC removal in Rakarrack. Understand that 
noise and poor quality sound is due to your hardware configuration, and 
it is probably not caused by Rakarrack.</p>
<p>5) Use Line In directly, but understand noise and poor quality sound is 
due to your hardware configuration, and it is probably not caused by 
Rakarrack.</p>

<p>If you have a guitar with a built-in preamp (active electronics), then 
plug directly into the sound card Line Input and all is dandy.</p>
<p><a href="help.html">Table of Contents</a> -  <a href="general.html">General</a></p>
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rakarrack 0.6.1-4build2 / usr / share / doc / rakarrack / html / hardware.html
