Which Audio spec ?

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    I'm looking at 2 new mid-tower cases. One has I/O jack that supports
AC97 audio and the other supports HD Audio. I'm confused as to which
one is most necessary? If it supports one, does it support the other?
(I've seen one with the following: 'supports AC97/HD Audio'). Would
this be 2 separate connections to the motherboard? I just listen to
cd's and mp3's. My granddaughter watches the occasional DVD. Thanks
for suggestions.

Re: Which Audio spec ?

On 7/13/2010 11:57 AM, Motor T wrote:
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AC97 is the old Intel header standard. HD is the new Intel header standard.
Both are 10-pin connections with 1 key position.


It seems that cases with front panel audio connections often have a
dual-purpose motherboard connector or two separate connectors so that the
case will work with either standard. All three of my recent Antec cases
have been like this.

 From a manual for a recent Antec case:

There is an Intel standard 10-pin ACí 97 connector and an Intel 10-pin HDA
Definition Audio) connector. You can connect either of them to your
motherboard depending on the specification of the motherboard. See
instruction below:

Note: Please check your motherboard manual for your audio header pin layout
make sure it matches the table below. Even if your system supports both
audio standards, you may only connect one connector not both."

Re: Which Audio spec ?

John McGaw wrote:
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    Thanks John,
    Very interesting reading..... I like the thought of having the PC
detect which port I plugged into and configuring the port to support
the device. (What will they think of next??). I guess I'll go with the
newer standard HD.
    As an aside, I like the Antecs but I'm afraid the features I want
(front e-sata port) are not available on their cases that are the size
I'm limited to. Thanks again.

Re: Which Audio spec ?

Motor T wrote:
Quoted text here. Click to load it

The answer to your question would be, the HDAudio defined tower, will
support any new motherboard, but would not be as convenient for a
ten year old motherboard.

The tower with AC'97 will work with older motherboards or new ones,
at least as long as the industry continues to support dual mode
operation of the FP_AUDIO header on the motherboard. All my cases
here are wired AC'97, and I do have a few HDAudio motherboards with
dual mode front panel wiring operation. And I haven't seen a rush
to doing HDAudio wiring only, at least when people ask this particular
question. I'd still seeing the motherboards having support for
dual mode wiring, and the cases are still mostly AC'97 wired.

It really should not matter which you use. If you have no interest
in putting ancient motherboards in that tower, then the HDAudio tower
should be fine. While the other one could be used for either purpose, old
or new.

Some cases come with two cable assemblies, and you can use the
one most appropriate for your motherboard (HDAudio cable assembly
for modern boards, AC'97 assembly for older ones.) All they're
doing in that case, is disconnecting a few wires from the AC'97


An older motherboard, had an AC'97 pinout which would work with your AC'97
case wiring.

Newer HDAudio motherboards may support both AC'97 and HDAudio *wiring* for the
front panel, but the trick there is, the AC'97 pinout definition is
"simplified", such that it is a subset of the HDAudio pins. In other words,
the AC'97 wiring on an HDAudio CODEC equipped motherboard, isn't the same
exactly as the AC'97 wiring on an older motherboard. That is how they
arrange them to share the same pins.

The end result is, the majority of computer cases support AC'97 wiring
(full or reduced definition). If you have a "solid block" 2x5 cable
assembly, rather than individual wire plugs, then pulling some of the
wires out of the solid block, to meet the comments in the middle
column below, is recommended. The individual wires scheme is more
flexible - you don't hook up the wires you don't need.

There hasn't been a complete switchover to pure HDAudio wiring. In part,
this was caused by the need for jacks with isolated side contacts, and
I didn't see any of those when I went shopping for some. They didn't seem
to be distributing them. I view that as a "chicken versus egg" situation,
where the case manufacturers didn't want to make any mistakes.

In terms of giving pinouts, I'm missing one of the documents I've used
in the past, and since the document was just a number, my odds of finding
it again are just about zero. In any case, these examples are to illustrate
the differences, if any.

Traditional AC'97      Modified AC'97            HDAudio
-----------------      (for dual mode)           (dual mode or otherwise)

MIC2        AGND       MIC2      AGND            Port1_L      AGND
MICBIAS     +5VA       MICBIAS   ???             Port1_R      PRESENCE#
FPOUT_R     RET_R      FPOUT_R   ---             Port2_R      SENSE1_RETURN
HP_ON                  ???                       Sense_Send
FPOUT_L     RET_L      FPOUT_R   ---             Port2_L      SENSE2_RETURN

(When not used, had    (No jumpers needed        (Port names reflect the
two jumper plugs, to    on header. No return      nature of the jacks. PRESENCE#
join FPOUT to RET.      signals are needed.       signal is grounded, if a real
HP_ON and +5VA were     Power removed for         HDAudio cable assembly is being
for an optional         non-existent headphone    connected.)
headphone amplifier.)   amplfier. Only five
                         wires should really be
                         connected, for fault
                         free performance! Leave
                         the return wires dangling.)

The "sense scheme" for HDAudio is shown on page 172 here.


The modern front panel audio wiring options are shown here.
Page 20 and Page 23. Page 20 is a form of modified AC'97,
but I wouldn't build a motherboard using that diagram
verbatim. Since Sense_Send shares the pin, you wouldn't
tie a hard +5V to that pin for example. I don't know
what Intel was thinking, when they did page 20. In any
case, you can see the inspiration provided by page 20,
reflected in the middle column above.


The older pure AC'97 pinout is shown on page 20 here for reference.
(This defines the older, leftmost column above.)


The three sense pins on the HDAudio are wired in the computer
case, to the two isolated side-contact switches on the
headphone and microphone jacks. They would look like this.
The front panel wiring assembly contributes the two switches to
this picture, while the resistors used to encode things are
on the motherboard itself.

                                   +--/  x--+
     --------------------+         |        |
                         |         |        X---- resistor --- GND
     CODEC   Sense_Pin ------+-----X
     chip                |   |     |        X---- resistor --- GND
     --------------------+   |     |        |
                             |     +--\  x--+
                             |         \
                         To two other jack
                         detection circuits on the

The front 1/8" stereo jacks have three contacts. The function and wiring
are summarized in this table.

                     TIP     RING     SLEEVE
Headphone           LEFT    RIGHT    GND
Microphone (mono)   MIC     MICBIAS  GND     (MICBIAS is weak power for
                                               and will not harm other things.)
Microphone (stereo) MIC_L   MIC_R    GND     (Separate bias is connected to
                                               both channels, to power stereo
                                               electrets, and will not harm other
                                               things. Example. Andrea



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