CS4236B Mixer
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The ALSA drivers use many more of the features of the CS4236B than did the drivers of days gone by. This gives you more flexibility in what you can do with the chip. But with the added flexibility comes added complexity, and sometimes a bit of confusion. This document will attempt to reduce that confusion.
Note the 'B' in CS4236B. This is not the same animal as the earlier CS4236. Although they use the same driver, the CS4236B is more than a minor revision of the CS4236, which had no capture mixer or wavetable input. If you have the earlier chip, much of this document won't apply to you.
Be forewarned that research for this document began as simple trail & error experimentation. However, I don't have the necessary hardware to try out the Synth, Wavetable, DSP, and Mono inputs. So I can't be sure if I have those documented correctly. I turned to the CS4236B specification to fill in the holes in my experiments.
Contents
The Basics
First, let's get some elementary stuff out of the way. If you are familiar with mixers in general and came here looking for specifics, feel free to skip ahead. But if you came here because you simply want to know how to set your controls to hear or record what you want, I'll try to provide a quick overview.
The following procedures assume starting with all switches turned off.
Listening
- Unmute the analog input that you want to listen to and adjust its fader (volume control). Analog inputs include Mic, Line, CD, Synth, Mono, and Master Digital. Huh? Master Digital is an analog input? Yes. Master digital is the mixed audio from the digital sources which has been converted to analog and fed to one of the inputs of the analog output mixer. If you want to listen to any digital source, choose Master Digital for this step and, if you hear no sound yet, leave the fader up at about 90%. The analog mute switches and faders often appear on the mixer application's "Playback" or "In" view. (The faders may be duplicated on a "Capture" view, but you are better off using the "Playback" view because the duplicate faders don't actually work in some mixer apps.)
- If your desired input in step one was anything other than "Master Digital", you have probably stopped reading by now, and are hopefully listening to glorious stereo sound. If, on the other hand, you want to hear a digital source, now is the time to unmute it and adjust its fader. Digital sources include PCM, FM, Wavetable, and DSP. The digital mute switches and faders often appear on the mixer application's "Playback" or "Out" view.
- It doesn't hurt to leave unmuted any or all of the sources that you normally listen to, and their faders adjusted appropriately. This saves you the trouble of going back to the mixer app every time you change sources. Leaving unused inputs unmuted could increase the background hiss slightly, but you may not even notice it during casual listening. However, it is best to mute unused inputs when making a recording -- no sense in saving that hiss for posterity.
If you are still not hearing anything, you may have chosen the wrong input. For instance, CD is the proper source to select if your player application uses the analog output from your CD drive. But some applications read the music digitally from the drive and send it to the PCM input of the mixer. Also, Synth may seem like the obvious choice for listening to MIDI files, but your soundcard may not have anything actually hooked up to the Synth input. The same goes for the Wavetable input. The only synthesizer inside the CS4236B is the FM synthesizer -- and even that will be silent until you load it with a utility such as sbiload.
Recording Without Monitoring
This method of recording doesn't let you hear the actual digital audio that is being sent to your recording application, so you won't hear any distortion that is occurring if you are overdriving the input mixer or the ADC (analog to digital converter). Therefore you should pay careful attention to the level meters in your recording application when using this method. Why use this method? It allows you to record from digital sources without introducing a feedback loop.
- For simple recording, make sure that Digital Loopback is muted. This will prevent a feedback loop. See Recording While Monitoring to see how to use this control to monitor the audio being recorded.
- Initially set Capture to 0% and Capture Boost to 100%. If you will be recording more than one input, you may need to drop Capture Boost.
- Unmute and turn on the capture switch for any analog inputs that you want to record and adjust their faders. Analog inputs that may be captured include Mic, Line, CD, Synth, and Master Digital. If you want to record one or more digital sources, choose Master Digital and, if you hear no sound from the digital sources yet, adjust its fader to about 90%.
- Unmute any digital sources that you want to record, and adjust their faders.
- The mix of inputs that you hear should be the same as what will be recorded (unless you have turned on one or more Capture Bypass switches or Mic Boost). But since you cannot hear the actual audio that will be recorded, it is important to pay close attention to the level meters on your recording application. Adjust the levels with Capture and/or Capture Boost.
Recording While Monitoring
This method of recording lets you hear the actual digital audio that is being sent to your recording application, allowing you to hear if any distortion is occurring. This works best when recording only analog sources.
- Ensure that the capture switch is off for Master Digital to prevent a feedback loop. This will prevent you from recording digital sources. (If you really, really want to use this method for recording with digital sources, feel free to experiment, but be aware that the feedback loop may add subtle (or not so subtle!) distortion to your recording. Listen for a tinniness effect being added to the sound.)
- Initially set Capture to 0% and Capture Boost to 100%. If you will be recording more than one input, you may need to drop Capture Boost.
- Unmute Digital Loopback and Master Digital, and initially set their faders fairly high.
- Ensure that all other inputs are muted, then turn on the capture switch for any analog inputs (except for Master Digital) that you want to record and adjust their faders.
- Adjust the levels with Capture and/or Capture Boost while watching the level meters on your recording application. If any distortion is occurring in the conversion from analog to digital, you should be able to hear it, and adjust Capture and/or Capture Boost to correct it. When using this method, the Digital Loopback and Master Digital faders will affect the level that you hear, but not the recording level.
Things Not To Do
Because the ALSA driver gives you so much control of the CS4236B's mixer, it allows great flexibility in what you can do. It also allows you to do some things that maybe you ought not to. This section discusses a few of the things you should probably avoid.
Don't unmute Digital Loopback when the capture switch for Master Digital is on.
Don't unmute Digital Loopback when the capture switch for Analog Loopback is on and Master Digital is unmuted.
Either of the above two items will create a feedback loop. If you are lucky, the levels will be high enough to blast you with feedback's familiar squeal, and you will immediately correct your mistake. If you aren't so lucky, the levels won't be high enough to make the distortion obvious, and only later when you are listening closely to your recording will you notice that things sound a bit tinny.
Don't turn on Line Capture and Line Capture Bypass at the same time.
Don't turn on Synth Capture and Synth Capture Bypass at the same time.
The capture bypass switches bypass the gain control for these analog inputs and send the audio signal straight to the capture mixer. If you also have the capture switch on, the signal takes two paths to the mixer, and strange things will result. You may notice a dead spot when adjusting the fader, where the sound gets softer and then louder again. It is likely that the signal is being distorted too.
The capture bypass switches are meant to be used in place of the capture switches to allow you to use the input faders to adjust the output volume only, without affecting the recording volume. (An audio purist might also point out that this also eliminates the possibility of any distortion being added to the recording by the gain control.) Obviously, if you use a capture bypass switch when recording multiple inputs, you must now depend on the other input faders to adjust your mix.
Details
In this section we get into the nuts and bolts of the CS4236B mixer. If you want to do something unconventional, the author hopes that this information will help you figure out how to do it.
Block Diagram
Here is a simplified block diagram showing the way the various mixer
controls inside the CS4236B relate to each other:
..........
+--- playback bypass--------->. .
| . .>---------------------- mono
| +- mute ---->. . out
| | ..........
| |
| +------------MONO OUTPUT ---+
| |
| .......... |
MONO ---+- vol ------------ mute ---->. . |
. . |
. . |
MIC ----- vol -+--- mute --- boost ->. . |
| . . |
| . . |
CD ----- vol ---+---------mute ---->. . |
| | . . |
| | . . |
LINE -+--- vol -----+-------mute ---->. .>----+-+--------------- line
| | | | . . | out
| | | | . . |
SYNTH ---+- vol -------+---- mute ---->. . |
| | | | | | . . |
| | | | | | . . |
| | | | | | +- mute ---->. . |
| | | | | | | .......... |
| | | | | | | |
| | | | | | +------------------------------------------+
| | | | | | | |
| | | | | | +---------- ANALOG LOOPBACK --+ |
| | | | | | | |
| | | | | | | ........... |
| | | | | | +- capture ->. . |
| | | | | | . . |
| | | | | +---- capture ->. . |
| | | | | . . |
| | | | +------ capture ->. . |
| | | | . . |
| | | +-------- capture ->. . |
| | | . CAPTURE .>- boost - vol -+----- pcm
| | +---------- capture ->. . | | out
| | . . | |
| +-----------capture bypass -->. . | |
| . . | |
+-------------capture bypass -->. . | |
. . | |
+- capture ->. . | |
| ........... | |
| | |
+--------------------------------+ | |
| | |
+------------------ DIGITAL LOOPBACK -----------+ |
| | |
| ........... | |
+- vol --- mute ---->. . | |
. . | |
DSP --- vol --- mute ---->. . | |
. MASTER . | |
PCM --- vol --- mute ---->. .>-- vol -+---------+
. DIGITAL .
FM --- vol --- mute ---->. .
. .
WAVETABLE --- vol --- mute ---->. .
...........
Control Description
Here is information on some of the controls, including the range for
each fader, a description of the control's use, and additional notes.
CONTROL: Master Digital
MIN: -94.5 db
MAX: 12.0 db
STEP: 1.5 db
USE: This controls the master level for all digital inputs (PCM,
FM, Wavetable, DSP, and Digital Loopback). Unmuting Master
Digital sends the audio to the analog output mixer. Turning
capture on sends the audio to the capture mixer.
NOTES: This works on both sides of the DAC, with a range of -60 db
to 0 db on the digital side, and a range of -34.5 to 12.0 db
on the analog side. Both sides combine to give an overall
range of -94.5 to 12.0 db.
CONTROL: PCM
MIN: -94.5 db
MAX 0.0 db
STEP: 1.5 db
USE: Controls the input level for the digital data coming in via
the ISA bus, usually from a an application that plays audio
(such as aplay).
CONTROL: Synth
MIN: -34.5 db
MAX: 0.0 db
STEP: 1.5 db
USE: Controls the input level for the analog input sometimes
connected to an external synthesizer.
NOTES: This is an analog input for an external source (such as a
Crystal Semiconductor CS9233 or a Yamaha OPL3LS), not an
internal synthesizer. The CS4236B's internal FM synthesizer
uses the FM control. If your soundcard has nothing connected
to this input, it does nothing.
CONTROL: Synth Capture Bypass
USE: This is an alternative to the Synth capture switch. It
bypasses the Synth input level control and so gives no
attenuation or gain.
NOTES: You should not use Synth capture and Synth Capture Bypass
simultaneously.
CONTROL: FM
MIN: -94.5 db
MAX: 0.0 db
STEP: 1.5 db
USE: Controls the input level for the digital data coming in from
the internal FM synthesizer.
NOTES: Before the FM synthesizer will favor you with music, it needs
to be loaded with instruments. The sbiload utility (in the
alsa-tools package) acts as a roadie for the FM synthesizer.
Try
sbiload --help
or
less /usr/share/doc/alsa-tools-*/sbiload/README
for details. (If the folks who assembled your eight-CD
distro didn't bother to find space for this important
utility, you can get it by downloading the alsa-tools package
from http://www.alsa-project.org.)
CONTROL: Wavetable
MIN: -82.5 db
MAX: 12.0 db
STEP: 1.5 db
USE: Controls the input level for the digital data coming in from
the wavetable serial port.
NOTES: This is a digital input for an external wavetable
synthesizer. The CS4236B has no internal wavetable
synthesizer. This control is only useful if your soundcard
has a wavetable synthesizer connected to the CS4236B (such as
a CS9236). And even then it won't be useful until the day
when (if?) ALSA supports such a wavetable.
CONTROL: DSP
MIN: -94.5 db
MAX: 0.0 db
STEP: 1.5 db
USE: Controls the input level for the digital data coming in from
the DSP serial port.
NOTES: This is a digital serial input for an external device. If
your soundcard has nothing connected to this input, it does
nothing.
CONTROL: Line
MIN: -34.5 db
MAX: 12.0 db
STEP: 1.5
USE: Controls the input level for the analog input typically
connected to a "Line In" jack.
CONTROL: Line Capture Bypass
USE: This is an alternative to the Line capture switch. It
bypasses the Line input level control and so gives no
attenuation or gain.
NOTES: You should not use Line capture and Line Capture Bypass
simultaneously.
CONTROL: CD
MIN: -34.5 db
MAX: 12.0 db
STEP: 1.5
USE: Controls the input level for the analog input typically
connected to a CD drive.
CONTROL: Mic
MIN: -24.0 db
MAX: 22.5 db
STEP: 1.5 db
USE: Controls the input level for the analog input typically
connected to a "Mic" jack.
CONTROL: Mic Playback Boost
MAX: 20 db
USE: Gives you microphone a 20 db kick on the way to the analog
output mixer. Does nothing for what goes through the Mic
capture switch.
NOTES: If your microphone is so tired that it needs a kick to be
heard in the capture mixer, you might try Analog Loopback
capture instead of Mic capture. (Remember to unmute the Mic,
and read the note for the Analog Loopback control to avoid a
feedback loop.)
CONTROL: Mono
MIN: -45.0 db
MAX: 0.0 db
STEP: 3.0 db
USE: Controls the input level for the monaural analog input, which
might or might not be connected to your PC's speaker output.
CONTROL: Mono Output
USE: Combines the analog output's stereo channels into a monaural
channel and routes it to the mono output, which might or
might not be connected to your PC's speaker.
CONTROL: Mono Playback Bypass
MAX: -9 db
CONTROL: Capture
MIN: 0.0 db
MAX: 22.5 db
STEP: 1.5 db
USE: This controls the master level for all inputs that have their
Capture switch set (Master Digital, Synth, Line, CD, Mic, and
Analog Loopback).
CONTROL: Capture Boost
MIN: -18.0 db
MAX: 0.0 db
STEP: 6.0 db
USE: This extends the range of the Capture control in 6 db steps.
NOTES: When mixing multiple analog inputs, it may be necessary to
reduce this control to prevent overloading.
CONTROL: Analog Loopback
USE: Turning capture on will route the analog output to the
Capture mixer. This is the only way to capture the Mono
input, and an alternative way to capture the other analog
inputs. But I suspect this exists mostly for compatibility
with software that was designed to interface with earlier
soundcards which had no capture mixer. On those soundcards
this was the only way to record multiple analog sources.
NOTES: If you have capture on for Analog Loopback, and unmute both
Digital Loopback and Master Digital, you will create a
feedback loop. Unless you want your dog to start barking,
this is probably not something you should do.
CONTROL: Digital Loopback
MIN: -94.5 db
MAX: 0.0 db
STEP: 1.5 db
USE: Unmuting Digital Loopback will route the captured digital
output to the digital mixer. If you are capturing analog
inputs and wish to monitor the digital audio being recorded,
you would mute the analog inputs and unmute Digital Loopback.
NOTES: Be sure that you have capture off for Master Digital when you
unmute Digital Loopback, or you will create a feedback loop.
(Hush now, Spot!)
Mixer Application Quirks
When used with the CS4236B, some mixer applications exhibit a few quirks. You should be aware of them so that they don't sneak up and bite you. The author is familiar with only these four mixer applications:
- amixer (version 1.0.10)
- alsamixer (version 1.0.10)
- gnome-volume-control (version 2.10.2)
- kmix (version 2.4)
All switches, except for two, on the CS4236B mixer are stereo pairs, allowing separate control of the left and right channel. (The two exceptions are Digital Loopback mute and Mono Playback Bypass.) Some mixer applications don't allow individual control of each channel switch, providing a single button for switching both channels. If you never have the need to switch the channels individually, this won't be a problem for you.
amixer
(No known quirks)
alsamixer
Six of the faders that appear in the "Capture" view are duplicates of faders in the "Playback" view. These duplicate faders don't work; use the faders in the "Playback" view. Only the Capture fader works in the "Capture" view.
Individual control of the stereo channels with the capture switches is supported, just not documented. Use the ; and ' keys for left and right, respectively.
gnome-volume-control
All five faders that appear in the "Capture" view are duplicates of faders in the "Playback" view. These duplicate faders don't work; use the faders in the "Playback" view.
There may be two Capture Boost controls on the playback panel. One works; the other is dead.
The gnome-volume-control has fake mute buttons for Capture and Capture Bypass. They merely set the levels temporarily to zero (without moving the indicators) and restore them to the indicated level when unmuted. Some may find these to be a convenience. Others will find them confusing, since they don't actually mute anything, if both are not "muted" sound can still be heard, and the levels can be raised externally while the indication is that it is "muted".
The gnome-volume-control mutes some controls when the levels are reduced to zero (Master Digital, Synth, Line, CD, Mic). It does not unmute them when the levels are raised again. Clicking the mute button when these controls are at zero will go through a three-phase cycle: unmute, unmute, mute.
There is no Analog Loopback control.
No individual switching of stereo channels (see above).
kmix
- No individual switching of stereo channels (see above).
Mixing Mixers
To avoid confusion, it is probably best not to have more than one mixer application running at any time. Normally there is no need, but if you need a control that your favorite mixer app doesn't support fully, go ahead and start another one. Also, some non-mixer applications (like music players) have faders that control the mixer. In theory they should all get along with each other, responding to your commands, and displaying the current state of the CS4236B's mixer.
However, some applications play well with others, and some don't. Here are some of the squabbles that you may have to mediate when running more than one mixer application.
As mentioned above, gnome-volume-control automatically mutes some controls when its fader is reduced to zero, forcing you to manually unmute them later. This is true not only when you use that app to zero the fader, but any app.
Whenever a left channel mute or capture switch changes, gnome-volume-control forces the right channel switch to match it. So even if you use a mixer app that can mute and capture individual channels, doing so can be confusing as long as gnome-volume-control is running. The work around is to set the left channel first, then fix the right channel if it changes.
If a control is muted, gnome-volume-control no longer updates its display of the current level of the fader. If, while the control is muted, another app sets its fader to zero, then later unmutes it and raises the fader above zero, gnome-volume-control will force the fader to jump to the position it was at when first muted.
The current state of the following five switches is displayed correctly when gnome-volume-control is launched, but they are never updated to reflect external changes: Synth Capture Bypass, Line Capture Bypass, Mic Playback Boost, Mono Output, Mono Playback Bypass.
As stated above, these conflicts only appear when running more than one application that controls the mixer. They are relatively minor quirks, but you should be aware of them so as not to start doubting your sanity when a control suddenly starts behaving differently than it usually does. When something odd happens, just ask yourself, "am I running another mixer app?"
This is a Wiki page, so feel free to update it if you have additional information to share, or see errors that need correcting. If you think that something might be an error, but you are not sure, you may want to add a comment to the CS4236B Mixer Comments page.
Retrieved from "http://alsa.opensrc.org/CS4236B_Mixer"