Sub Out Board?

Anyone interested in a sub out board? Apparently the necessary software mods are now a possibility. What features would everyone like one to have?

Stu

Hi.

I would certainly be interested. Just an RCA (4V) output would be enough.

cu,
sven

Where would you put it if you have the digital out board installed?

Stu

Good question. I would probably put the board where the second drive would be and route the output through the mic input wires.

cu,
sven

How about a user selectable, adjustable low pass filter and volume control accessable from the empeg menus.

That's fine, but it's out of my hands. That would involve programming in the kernel or player software for which I have only the most basic knowledge. The hardware, I can do. The software is another matter.

Stu

I would love a board similar to the digital out board, so that I can keep my second drive in there.

TommyE

Yes! I'll order one!

Marius (Escort Cab + Mark II)

Count me in for one.

Dave

I'd definitely want one.

Would this have a DSP'd crossover or would it just be a fifth (or third) output?

SInce you asked for a wish-list:

I'd love to see a DSP crossover.
Possibly a seperate Bass volume/level control.

I realize that these are again probably kernel hacks but they'd be nice to see. I'd be in for one.

Yes, the plan is to have the DSP handle the crossover.

Stu

Id be interested... any (rough) estimates on cost?

Maybe $80. I don't know. All it would be is a very small single channel DAC circuit, so it wouldn't be very complicated.

Stu

well, sure, maybe someone else can step up to that job.

Well, could the preamp and DAC be fitted on your digital out board? and then you can wire it to a common phono cinch?

I think you'd have to put a stereo DAC on there (well, it'd probably cost less that way) as the centre & sub are on left & right of that stream if configured appropriately. There's something like a 4th order low pass for the sub output, and the level would all be adjustable in software - all that'd be needed is a buffer opamp to (a) protect the dac and (b) give a decent drive level.

Hugo

Yes, it would be stereo, probably a CS4334 to keep size down. The sub signal would be mono (same data on both channels) would it not be? So there would only be one channel of output and one output stage.

Stu

It would require major rework. Don't expect to see the two integrated any time soon, if ever. Count on them being separate.

Stu

hrm.. i think i woudl order one, and still get the other board, one for each empeg

Would it be hopeless to get both boards inside one Empeg without taking up HD space??

TommyE

In a word, no.

Stu

Didn't you mean "yes" (re. the double negative in the previous post)?

So it will not be hopeless??


TommyE

It is probably possible to get both boards into the Empeg simultaneously.

Stu

There will be 2 RCA out's, even though it is mono??
Just asking as my amp. has a subwoofer input which is mono, but 2 RCA's.

TommyE

The DSP outputs a centre channel and a subwoofer channel. They're the left & right on this extra I2S output which has to be enabled. So depending if you want to support the centre channel or not is if you want to add an extra output stage.

- Trevor

So the subwoofer channel has a Xover of some kind, opposed to the center which is just straight thru????

If so, it would be cool with both signals out.

TommyE

I thought that, in a car stereo, the proper configuration for a center channel speaker is to only play the difference between the left and the right channels, not the sum of the left and right channels.

Summing the two channels for a center channel would simply worsen the sound stage, whereas a difference channel would widen and improve it.

So basically sub would be on one of the channels and center would be on the other? I'll have to see if there's room to have two output stages. If someone writes a kernel mod before I can throw together a circuit please let me know to which channel the sub and center are routed.

Stu

I may have read your post wrong but I thought the centre channel comprised of stripping out the differences and only allowing through what's common to both channels. Otherwise you'll get all those nicely separated signals coming from the centre of the soundstage and stuff like vocals (which are usually placed in the centre) removed completely. This is how it works isn't it?

In reply to:

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I thought that, in a car stereo, the proper configuration for a center channel speaker is to only play the difference between the left and the right channels, not the sum of the left and right channels.

---

I don't think that's what he means. I believe he is saying that one of the channels on the I2S carries the DSP generated center and the other, the DSP generated sub.

Stu

With a center channel, is true 5.1 channel audio possible? Or would we need more than just software to decode it? Of course we would probably have the same issues as getting OGG support, unless the empeg gods smile upon us, and the player software becomes open source... I know I know... I'll never live to see the day. But would the 5.1 audio be possible?

I may have read your post wrong but I thought the centre channel comprised of stripping out the differences and only allowing through what's common to both channels.

Okay, I think that's what I meant.

If you summed the two channels, you'd get everything as if it were a mono signal (including things only on the left channel for instance), so that's not what you want.

If you subtracted the two channels, you'd get only the stuff that was unique to the outside channels, which is also not what you want.

I don't know the exact mathematical operation that would result in "only the center channel sounds appear in the center", but simply summing the two channels would suck. Does the DSP simply sum the two channels to get the center channel, or does it Do The Right Thing?

With a center channel, is true 5.1 channel audio possible?

Negative.

5.1 audio requires a special kind of digital datastream which can't be read or decoded by the stereo-CD-player-targeted DSP. I don't even think you could do it with the digital output board because all the 5.1 decoders expect a different samplerate than what our player delivers (48 instead of 44 if I recall correctly).

Not even matrixed Dolby Surround (4 channel: L, R, Center, RearSurround) could be done, because there's no Surround decoder on the DSP chip, and its only input is stereo, so you couldn't do Dolby decoding in software either because there would be no place to send the other two channels.

In reply to:

---

I don't know the exact mathematical operation that...

---

Hmmm... I've been trying to get my head around that one too .

Thats what I was afraid of. Thanks for the quick reply!

Hmmm... I've been trying to get my head around that one too.

Well, let's see. The way I'd do it (I think) is...

Phase-invert one of the channels and subtract it from the other channel. This will give you a track that's only the outside two channels. Then take THAT and subtract it from a mono-summed version of the original signal. The result is just the stuff that's common to both channels.

Note that the first half of the equation (phase-invert and subtract) is how most of the "vocal eliminators" work. They assume that the voice is going to be mono center, and the instruments will be panned L and R.

5.1 audio requires a special kind of digital datastream which can't be read or decoded by the stereo-CD-player-targeted DSP. ...

Not to mention that mp3 and wav only support two channels. Ogg Vorbis can support more, but no current implementation does.

Page 108 of the 7705 manual (as it is noted in the PDF itself) covers the issue of how the center and sub signals are derived. There is some sort of illustration on that page but it won't load it properly, so I can't really find the answer, but it does say that the "the center signal is derived from the sum signals of the front channels just in front of the Soft Audio Mute coefficient." The sub signal is the sum of all four channels and is output on the left portion of the I2S secondary, while the center is output on the right portion of the secondary I2S.

Stu

I'm not sure that would work. If you phase-invert one of the channels and then subtract it from the other one aren't you just going to double the amplitude of the bit you're trying to get rid of? ie you've got a point on the waveform which has a value of 0.3 (we won't worry about units they're irrelevant) if you invert this you get -0.3. If you take this away from the corresponding 0.3 on the other channel you'll get 0.3 - -0.3 = 0.6
It's getting too late for all this brainwork (it's 02:25 here) I'm off to bed. Hopefully someone will have figured it out by the time I wake up. Then we'll all say how obvious it is .

Yeah, I have seen Dolby Digital sound files however.. some came on my CD with my Sound Blaster Live! So there is the format out there. I don't remember off the top of my head their extension, but I had to use the Creative PlayCenter to listen to them.

The Mathematical term is (I believe) The "Intersection" of the two channels. The symbol is an upside-down "U".

Marius (Escort Cab + Mark II)

My point was that the empeg doesn't support any formats that have more than two channels, so it's a rather moot point.

Is anyone able to properly view the entire 7705 manual? I keep getting an error on page 108 (among others) regarding MSDRAW which is preventing me from seeing the figures explaining the sub and center channel functionality.

Stu

The error I get on that page is not related to MSDRAW.

It says it cannot find the FONT named "MS Line Draw". I suspect you only need to locate a copy of that TrueType font on the net somewhere and install it on your system.

Yeah I just checked, it's MSLINEDRAW. That cleared up the problem. Thanks

Stu

For others missing the MS LIneDraw font... I have attached a copy to this post...

Stu:
I assume you are currently using IISOUT1 (pin 34) for your digital card. Do you know if IISOUT2 (pin 35) connects to anything external on the DSP?

It does, it connects to the IIC output header for just this sort of reason.

The only design flaw does appear to be that we forgot to put the MCLK (256fs) on the same header. Oops.

Hugo

So Hugo...

Stu does connect IISOUT1 to his digital out board

OR

There is a connection for IISOUT2 on the header

OR

both?

Shonky, could you please post another copy of this file? When the BBS move happened, a couple of the most recent file attachments did not make the move, I assume yours is one of them.

Unless the resulting ZIP file was over 100k, in which case, the attachment will never work.

Here it is.

Both IISOUT1 and IISOUT2 pins on the DSP are connected by traces leading to an area marked as "IIS" on the Empeg motherboard. Besides IIS1 and IIS2 (actually SDATA signals), this output header contains a pad with LRCLK, SCLK, and ground. I believe Hugo meant that the MCK pad (the 11.2896 MHz oscillator signal which is 256 times the 44.1 KHz sampling rate of the Empeg hardware) was inadvertently omitted. from this "IIS" header. That header is not labeled "IIC" despite what Hugo says. The digital out board has a ribbon cable which connects to IISOUT1 as well as LRCLK, SCLK (we get ground from a screw rather than the ground pad because it was tough to solder to), and directly to one side of the 11.2896 MHz crystal. The sub out board would use IISOUT2 rather than IISOUT1, but all other connections would be the same.

I have attached a picture from our soon to be completed manual to illustrate.

Stu

Sorry, brain fade. I did mean IIS not IIC...

Hugo

OK, Well I've done the code for subwoofer and centre channel output.

Subwoofer has a 40, 60, 80, 100 or 120 Hz 4th order filter with gain from -20 to +24db. Centre channel has a gain of -20 to +24db.

So Stu, I've done the software, have you got any hardware? ( just jokes). I am going to build that hardware we discussed to test it hopefully this weekend.

I've just been looking at the empeg CPU card. I hope your pictures aren't as cropped in the install manuals. The part showing the crystal can be misleading since the crystal can be soldered to the board either way around. Only if you look very very carefully at the vias can you make out which way is up. My board happens to have the crystal the other way around. A less cropped view will obviously show which side is which with the DSP and the 6MHz crystal on either side.

Also are you sure that side of the crystal you are connecting to is the OSC_OUT (pin 64) side? i.e. is it the side driven by the internal oscillator. I'm not saying you're wrong, just check you have it right.

I hope your pictures aren't as cropped in the install manuals.

No, what I posted was about 3% of the frame (I told you they were high-res ) The manual will show the full frame to make it easy for people to get their bearings.

Also are you sure that side of the crystal you are connecting to is the OSC_OUT (pin 64) side?

I never traced it out. I tried both sides actually, in an effort to avoid using a buffer, but I got the same results from either side, so I didn't really worry about it. After adding the buffer circuitry, the output from the digital out board is rock solid, so I don't think there is anything wrong with my approach.

Stu

Don't they perform something similar to create a center channel in the "old" Dolby Pro Logic : (L+R)-(L-R) ? I beleive this is what Jane is referring to with the mathmatical intersection (sum minus the difference).

Hi there.

I really like the idea of adjusting the sub output from the empeg - I'm sick of diving behind the back seats of my car to tweak the sub amp whilst waiting at traffic lights and stuff.

I think I'd like to buy one of these (probably as a kit, coz I like a challenge) but I'm wondering about the acutal connections from the empeg to the outside world....

Are there are spare pins on the car sled connector to allow an analog bass output to be passed through? Any kind of trailing leads flowing from the back of the empeg are going to be a real pain to connect and would probably foul when trying to dock the empeg...

Anybody have any thoughts on this? Are there any relatively unimportant pins that could be reused, or would it involve fitting a new custom connector to the empeg & the sled?

Looking forward to hearing more news on this project....

Cheers,

Andrew.

There are spare pins, if you don't want the use of the mic in line for its intended purpose. It would be quite simple to route the sub signal out of this cable using a minijack to RCA adapter.

However, if you also want a center output as well, you will run into trouble I think. There is only one mic in line after all.

Stu

It would be quite simple to route the sub signal out of this cable using a minijack to RCA adapter.

Could you ellaborate on this, or point me to something that will explain how to do it. I'm looking to add a sub before the Cinncinnati meet, but most of this thread is beyond my level of tech speak.

Well, find the wire(s) (inside the empeg) going to the mic connector. Disconnect it/them from what it/they're currently connected to, and connect it/them to the sub out board instead. Then get an 3.5mm mono (?) jack to RCA adapter. Plug adapter into mic connector, RCA wire into the resulting RCA jack. Other end of RCA wire to sub amp.

Couldn't be easier

Of course, getting a sub out board before the meet (forgot when it's planned) might not be realistic....

/Michael

Thanks...

Okay. Open the player. In the rear left corner of the MK2s are two rectangular connectors. One is labeled "Analogue" and the other "Digital."

Pull the one labeled analogue from the board. Use a very fine flat bladed screw driver and remove the gray/black striped wire from the connector located on the far left of this connector. This wire will carry the sub signal across the mic in cable. I don't remember what color the ground half of the cable is, but you can get ground from almost anywhere. Connect the sub board output cable to that pulled wire by some means such as soldering or some sort of connector (what I will be doing). Connect the ground half of the output from the sub board to the cable ground. Put everything back together and connect your cables using an appropriate adapter. Enjoy.

DISCLAIMER. This ia all from memory and I've never actually done this, so this post could be riddled with errors.

Stu

Of course, getting a sub out board before the meet (forgot when it's planned) might not be realistic....

Not likely to happen by 9-21, unless he learns how to design and build one himself. Digital out boards yes . They will be ready before then.

Stu

I'm looking to add a sub before the Cinncinnati meet, but most of this thread is beyond my level of tech speak.

Keep in mind that you probably won't be getting your hands on a working sub-out board by the time of the Cincinnati meet, so you will need to wire up a sub the regular way. Please click here to see details on the regular way.

OK, according to page 108, the centre channel is simply the sum of the Front Left and Front Right channels divided by 2 after the fader coefficients have been applied (i.e. fading to the back will decrease the volume of the centre channel.

The mono subwoofer channel is calculated similarly and is (FL + FR + RL + RR)/4 but taken from before the fader coefficients so they don't affect the subwoofer level. After that a 4th order low pass crossover is applied.

If using left and right subwoofer modes (no centre channel available) the left subwoofer channel is (FL + RL)/2 and the right channel (FR + RR)/2. The low pass filter is only 2 order though for each channel if you use stereo subwoofers.

So they are simply averages according to the doco which I dont see being much problem on the subwoofer channel but I can see being an issue with the centre channel - constructive and destructive interference will be happening I think.

I tried both sides actually, in an effort to avoid using a buffer, but I got the same results from either side

Well the side you used is the OSC_OUT pin which is what you should be using so it's all good...

I can see being an issue with the centre channel - constructive and destructive interference will be happening I think.

Only one way to find out. . . Who needs stereo subs anyway?

Stu

I'm not worried about stereo subs.

Where did you take +5v from? I almost have my DAC wired up so I can find out now.

There is a small jumper under the 2nd drive bay labeled "+5." That's where it's coming from. Would you like another picture (uncropped this time)? Good job on the DAC.

Stu

Cool. I was looking for a place to solder though so I soldered to the inductor connected to that jumper.

It works fine. Sound quality is good considering I'm not using anything other than the DAC to drive the auxiliary of my home stereo. I need to hook it up to a decent system to test it for sure. I will probably connect it to my PC sound card and do a spectral analysis of some pink noise to see the subwoofer roll off.

My kernel stuff all seems to work OK too - only prob is a slight thump when changing the crossover frequency to a lower value (i.e. 100 to 80 Hz but 80 to 100Hz is fine)

There is however some noise getting in somewhere when I turn the gain of my amp right up (at normal levels you can't hear it), but I would guess that it is getting in on my breadboard (or the power supply lines) more than anything. Sounds kinda 50Hz-ish so I think that will go. I will do some PCBs next week some time.

In all an easy job - I only spent about 5 or 6 hours from whoa to go. I can put up some quick pics of the hack on Monday...

Glad it worked. I went with a jumper because I like to minimize permanent modifications to the player as much as possible. Even if I unsoldered the connection it would still show, hence the jumper.

I will be doing my own design although not for a few weeks. Emphasis will be on small size and sound quality. Did you test the center output? You put no circuitry of any kind between the output pins and the stereo?

Stu

Glad it worked. I went with a jumper because I like to minimize permanent modifications to the player as much as possible. Even if I unsoldered the connection it would still show, hence the jumper.

Yeah but you soldered to other parts of the board that will show so what's the difference. I also didn't have a suitable jumper to use.

Did you test the center output?

Yes it works fine - easy to test since it's simply the other channel of the DAC. A little hard to tell how "centred" it is though.

You put no circuitry of any kind between the output pins and the stereo?

The only circuitry I have after the DAC is the RC network on page 10 of the CS4334 datasheet. They are after all line level outputs. I believe this CS4334 is designed to be used like this without any further major circuitry after the DAC. The eval board even has outputs like this although it does have another circuit with an active output stage

Stu,

are you doing a center channel output also??? If you do, you're going to end up making me gouge apart my dash to install a center!!

Calvin

Well given that the centre channel out is simply the right channel of the digital I2S output and the DAC will probably be stereo, it's simply a question of having the output circuitry and the connector.

However I think Stu is planning on using the mic connector so it can be used in car on the sled which would rule out having two channels. You could however forgo the sub out for a centre out.

For the sake of size, I think it would be most sensible to make it sub out only. An ultra high quality output stage would be less important at these frequencies and having only one channel of analog circuitry would allow the board to be significantly smaller, but we'll see what turns up this week.

Stu

We built a DAC board for sub/center out. Something's not right with the digital signals. It almost acts like it needs another stage of buffering. When connecting it to the I2S output on my Rio it outputs terribly distorted sound unless I bridge the MCK input to ground with my finger. Then it sounds pretty good. There's nothing wrong with the I2S outputs. Those were tested and provide pristine sound through a P-3A DAC.

It turned out pretty small at only 11/16" x 1 1/16". The output is selectable as sub or center via a jumper. No time to work out the bugs at the moment. Hopefully soon. Here's a poor quality pic.



Stu

What? You mean your finger won't come with the kit?

Do you have any bypass capacitors for the DAC? I've had a few that kept going nuts from the interference from the switched mode PS until I put some bypass caps in close to the chip.

You could always fudge it by putting in a resistor + capacitor to simulate the loading on the clock line due to your finger

- Trevor

The bypass caps are there. I thought about loading the line down too. That's next. Right now I have another little project that needs my attention . Thanks for the suggestions though.

Stu

Whatever was bothering the board seems to have corrected itself. It works reliably and admirably now. The only change is that I rewired the digital out board (necessary to buffer the I2S signals) to digital supply and ground from analog supply and ground. Not sure why that would help.

What do people think would be a good voltage output level for this thing? I can go pretty close to five volts, but no higher. I will be ordering parts for an improved version pretty soon, so I need to figure out what value resistor I need for the gain to achieve the desired output level. So far one vote for 4 volts. RMS or P-P? The caps in the analog path will be poly, rather than ceramic and resistor will be precision .1% instead of 1% so so resistors. I may need to experiment with layout too in order to get the best sound.

So far highs are not real extended and the noise floor could be better, although it's better than many stereo components already. More updates to follow.

Stu

wouldn't it be safe to set it the same voltage output as the other line outs? isn't that the way most head units with sub out work?

Okay, then I guess the output will be 4 Vpp. That is what the Empeg hardware outputs right?

Stu

I finally ordered the parts (nice metal film .1% 10 ppm SMD resistors and PPS SMD caps) to improve the sound quality of the sub board and to increase output to 4 volts P-P. I might change the layout or circuit a bit too if that doesn't help enough.

Stu