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How On-Board Audio WorksBy Gabriel Torres on February 2, 2008

Introduction

Today all motherboards have an embedded sound card (this feature goes by several different names, likeon-board audio, on-board sound, integrated audio or integrated sound! "n this tutorial #e #ill e$%lain ho#

the on-board audio is %roduced, allo#ing you to understand #hat a codec is and #hat its im%ortance tothe audio &uality is!

'ome very high-end motherboards dont come #ith the audio section on the motherboard, but in add-oncard that comes #ith the %roduct, being installed on a )*" +$%ress $ slot or on a s%ecial connector on the

motherboard! This o%tion is used only because some high-end motherboards have already so manyconnectors on the rear %anel of the motherboard that the audio connectors dont fit there, and using an

add-on card solves this %roblem, as the connectors #ill no# be available at one of the slots of thecom%uter case!

udio can be available in t#o different formats. analog or digital! *om%uters are digital systems so theycan only %roduce and mani%ulate audio in digital format! The %roblem, ho#ever, is that in the real #orld

audio is an analog entity! '%eakers are e$%ecting an analog signal so they can re%roduce sounds/ youcant feed s%eakers #ith a digital signal the so called 1digital s%eakers are in fact analog s%eakers #ith

a digital-to-analog (3* converter converting the digital signal sent by the com%uter into analog signal!4n motherboards there is a chi% called codec (short for coder5decoder that is in charge of converting

digital audio signals into analog and vice-versa! This com%onent is very im%ortant as it defines the audio&uality of a sound card, and #e #ill talk a lot more about it later!

The %rocess of converting the digital signal sent by the com%uter into an analog signal, so you can hearsound on your s%eakers for e$am%le, #hen you %lay an 6)7 file or #hen you %lay a video file is called

digital-to-analog conversion or 3* for short! The inverse %rocess, i!e!, converting analog sounds sent tothe com%uter through a micro%hone or through its 1line in in%ut into digital for e$am%le, #hen you

hooku% a ta%e deck or a turntable to your )* to convert old music into 6)7 files is called analog-to-digital conversion or 3* for short!

4n any sound card including those embedded on motherboards you can find t#o ty%es of connectors.analog and digital! nalog connectors (usually 7! mm mini 9acks allo# you to connect your sound card

directly to s%eakers (i!e!, 1analog s%eakers! This is the chea%est and easiest #ay to connect s%eakers to

your )*!

3igital connection, also kno#n as ')3"F ('ony5)hilli%s 3igital "nterconnect Format can be found in t#oflavors, coa$ial (using a mono :* connector or o%tical (using a connector called Toslink! This

connection allo#s you to connect your sound card to home theater receivers and digital s%eakers! s #ehave already e$%lained, louds%eakers are analog devices! ;ome theater receivers and digital s%eakers

have a digital-to-analog converter inside that converts the digital signal received into analog and thensend the signal to the s%eakers!

3igital connection %rovides some advantages com%ared to analog connection! First, usually home theaterreceivers and digital s%eakers use a better codec than the one used on the motherboard and because of

that the audio &uality is higher (lo#er noise level, mainly! 'econdly, home theater receivers and digitals%eakers may %rovide features not found on analog s%eakers, like 3olby )ro

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The dra#back of digital connection is %rice, as the com%onents involved #ith digital connection are moree$%ensive (the cost of a home theater receiver and a set of s%eakers is far higher than the cost of a set of

analog s%eakers for your )*, the reason being the use of a more e$%ensive codec and also the cost of thedecoders for several additional enhancements, like the abovementioned 3olby )ro

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click to enlarge

Figure 3: nalog audio connectors!

The number of analog connectors you #ill find on your motherboard #ill de%end on ho# many audio

channels your motherboard has (2, ?, @ or 8 by the #ay, 1channels means 1out%uts for individuals%eakers, and ! and @ are synonyms, as A! and 8 are also synonyms/ these are different #ays of

saying the same thing! 4n the best case (eight channels a!k!a! A! format you #ill have si$ connectorslike %ortrayed in Figure 7! The color code used by analog audio 9acks is the follo#ing.

)ink. 6ic in

Blue.

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second #ay #hich no#adays is only seen on fe# very high-end motherboards is using a dedicatedcontroller to control and %rocess the audio and thus not using the system *)= for these tasks!

But both the south bridge chi% and dedicated controllers arent ca%able of dealing #ith analog audio, only#ith digital audio, re&uiring an e$ternal chi% the codec to %erform the interface bet#een the chi% and

the analog connectors!

The south bridge chi% also called "*; or "54 *ontroller ;ub by "ntel controls the ma9ority of %eri%heral%orts located on the motherboard, like the ='B %orts and the $ )*" +$%ress connections! "t is a big chi%

and usually it is located far a#ay from the *)= on the motherboard, normally #ith a %assive heatsink onto%! The big chi% near the *)= is the north bridge chi%!

click to enlarge

Figure 4:

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click to enlarge

Figure 6:'outh bridge chi% #ith its %assive heatsink removed!

"n Figure A, you can see %art of the block diagram of a south bridge chi%! 'ince this block diagram #as too

big, #e are only sho#ing the %art that is of interest for our e$%lanations. the audio interface (1"ntel ;igh3efinition udio in Figure A! This block diagram is from an "ntel "*; south bridge chi%, used by several

mainstream chi%sets like )@ and )7 (and %ortrayed on the %ictures above!

click to enlarge

Figure 7:Block diagram of a south bridge chi%!

s you can see the audio interface is very sim%le, %roviding only a fe# %ins, #hich are connected to theaudio codec located on the motherboard!

The Audio Codec

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The south bridge chi% or the audio controller cant deal #ith analog audio! They need a small chi% calledaudio codec (short for coder5decoder to make the %ro%er digital-to-analog (3* and analog-to-digital

conversions (3*! 3igital-to-analog conversion is made #hen the com%uter is sending sounds to thes%eakers, #hile analog-to-digital conversion is made #hen you are feeding the com%uter #ith an e$ternal

analog audio source (for e$am%le, #hen you connect a ta%e deck or a turntable to the )* to convert musicinto 6)7 or *3!

)hysically s%eaking the audio codec is a very small chi% measuring C s&! in! (A mm2 and usually locatedon the rear border of the motherboard (see Figure 8! The t#o most %o%ular manufacturers of this chi% is

:ealtek (:T*, #hose chi%s are named starting #ith the letters

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click to enlarge

Figure 10:nalog 3evices 388B codec!

"n Figure #e sho# you a small diagram e$%laining the relationshi% of the south bridge chi%, the codec

and the audio connectors found on the motherboard!

click to enlarge

Figure 11:;o# the on-board audio #orks!

Audio u!"it#

'ince the codec is in charge of the digital-to-analog and analog-to-digital conversions, the &uality ofanalog audio de%ends e$clusively on the &uality of the codec used on the motherboard! There are several

%arameters that can be used to 9udge the &uality of an audio codec! Belo# #e listed the main %arametersthat you should use to com%are codecs (and thus the &uality of the on-board audio of a motherboard and

the most common values for each %arameter! The higher the number, the better!

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*hannels. 2, ?, @ (!, 8 (A! or 0 (8D2!

:esolution. @-, 8-, 20- or 2?-bit!

'am%ling rate. ??!, ?8, @ or 2 k;E!

'ignal-to-noise ratio (':. 80 to over 00 dB!

"t is im%ortant to note that several codecs offer one value for its in%uts and another value for its out%uts for e$am%le, offering u% to 2 k;E sam%ling rate for its out%uts but only u% to @ k;E to its in%uts (#hich

is the case of :ealtek

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nd finally #e have signal-to-noise ratio, also called ':, #hich measures the noise level %roduced by thecodec! "n our o%inion this is one %arameter that average users dont %ay a lot of attention, but they

should!

>ee% in mind that #hat #e are talking here is the noise %roduced by the codec, not the noise that is

already embedded on the audio source! This %arameter has nothing to do #ith the noise that is alreadyinside a video file or a 6)7 file that you may have!

To understand #hat #e are talking about, make a sim%le e$%erience! t home, find a sound system that

still has a ta%e deck! )ut a ta%e to %lay, %ress the %ause key and increase the volume! Iou #ill hear a lotof noise (technically called #hite noise that is being %roduced by the ta%e deck and isnt inside the ta%eyou are %laying (as the ta%e is sto%%ed! That is the noise #e are talking about! 4f course you dont #ant

a motherboard that inserts noise on your sound!

The signal-to-noise ratio is measured on a unit called decibel (dB! The higher this number, the better!

"deally you should have a motherboard #ith a codec that has a signal-to-noise ratio of at least 00 dB forthe best audio &uality %ossible! 6ainstream codecs offer a signal-to-noise ratio of dB or A dB, #hich is

1almost there and #ill %rovide a very good audio &uality for verage Joe!

The %roblem, ho#ever, is the signal-to-noise ratio for the audio in%uts! The signal-to-noise ratio advertised

by the motherboard or codec manufacturer is for the codec analog out%uts only (3* ':!