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Pls help me understand digital(out) multi channel audio interface

ceramic

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hello everyone, hope you're doing well.

i am trying to understand how a multichannel digital signal can be generated from a stereo audio file, and what software & hardware is needed.

my goal is to feed a digital 2.1 signal into a receiver from a computer (music only, currently foobar2000 as a player) thus bypassing the subwoofer crossover of the receiver.
(similar to using the multichannel analog input on a receiver, but using the coax or optical input, such as playing a surround dvd where each channel is already preset)

as i understand, using an interface w/ usb input and optical/coax output transmits the bits from the pc to the receiver w/ no digital to analog conversion taking place.
i also understand i could use an EQ on the pc, BUT could i also split the signal into L +R + subwoofer (software crossover)?
do i need a multichannel interface for that in addition to some "crossover software" installed on the pc?
or is there a product capable of "generating" a 2.1 digital output signal (w/ the option to set the crossover point) when playing stereo files?

apologies my lack of knowledge/terminology, i hope i managed to explain myself and i would really appreciate some help in getting educated on this topic.

thank you very much, wish you all all the best.


=========================================================================================
extra clarifications:
i am currently using a behringer uca202 to feed a digital signal 2.0 (stereo L+R) to the receiver and then use an analog EQ/crossover (from the analog receiver L + R signal outputs) to feed an analog signal to a sub. the receiver acts as the DAC, the uca202 acts as an USB input, optical output interface between the pc and the receiver, the EQ/crossover acts as the low pass filter for the subwoofer.

i do not use the receiver's or subwoofer's crossovers for two reasons: i need both a lower crossover point and a steeper filter than what receivers offer (i have a collection as a hobby).

i am aware there are analog solutions for my needs (such as minidsp, motuM4, the ultracurve deq2496, etc) but i would really enjoy using a digital signal with my preferred subwoofer crossover point included and have the receivers do the decoding (L,R, Sub) and the digital to analog conversion.
 

digitalfrost

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my goal is to feed a digital 2.1 signal into a receiver from a computer (music only, currently foobar2000 as a player) thus bypassing the subwoofer crossover of the receiver.
If you are okay with using a PC as the only source, EqualizerAPO under Windows can do that for free (if you are using DirectSound/WDM). You need any kind of multichannel audio interface. Windows supports up to 8 channels using WDM.

You can either buy an interface with integrated DACs or get something like MiniDSP UDIO-8 or RME Digiface to feed dedicated DACs. From these you just feed power amplifiers. If the receiver you already have can accomodate that I don't know.

Volume control should happen through the PC then.

My personal setup is a RME Digiface. I have closed box satellites (falling at 12db/oct below fs, -3dB at 80hz) that I highpass at 80hz with 12db/oct to reach 24db flanks for the highpass. For the subwoofers then, I use a 24db/oct lowpass. This feeds into two Topping D50 DACs, one goes to a Benchmark AHB2 (Sats) and one to an XTZ Edge A300 (subs). I think this is more or less what you want to do.

I have to say, it would be cheaper to get a good multichannel audio interface in the first place instead of dedicated DACs. However, analog connections between computers and amplifiers without using balanced connections (XLR) are prone to ground loop issues which was what started me on this path in the first place, since the Digiface allows me to have galvanic isolation between PC and audio components. That said, nowadays I would not buy any analog gear without XLR inputs.

If your receiver has a 3-pin PSU plug I would expect problems when using analog RCA connectors between your PC and the receiver.

e: Also note that Topping just announced a rather affordable highend multichannel DAC with balanced outputs: https://www.audiosciencereview.com/forum/index.php?threads/topping-dm7-8-channel-dac-review.35661/
 
Last edited:

zermak

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@ceramic
Before buying anything give EqualizerAPO a try: if you already have a desktop/tower PC you can use its on board audio sound card/DAC (the 3,5mm jack outputs on the mainboard behind the case; which is probably a 5.1 or 7.1 version) to experiment with it and see if it suits your needs.
I am actually using it to cross my fullrangers with a cheap sub and also using it to equalize the in room response (plus downmix/virtual surround while watching movies/TV shows with my headphones). It's a neat and powerful software and you can use it for every kind of DSP.
I think there are a few discussions about how to use it in the forum, a search should do the trick :)
 
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ceramic

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thank you for the reply.
i will head over to "EqualizerAPO" webpage and educate myself a bit.

so as i understand, the audio interface HAS to be multichannel even if i only use its digital output?

ps: volume control on the pc would be inconvenient, also i have no real need for a separate DAC. receivers have a satisfactory DAC, already paid for too :}
 

digitalfrost

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so as i understand, the audio interface HAS to be multichannel even if i only use its digital output?
Yes. EqAPO cannot copy or synchronize between multiple interfaces so you have to use a single interface that provides all the channels needed.

ps: volume control on the pc would be inconvenient, also i have no real need for a separate DAC. receivers have a satisfactory DAC, already paid for too :}
How will you then have the satellites and subwoofers at the same volume?
 
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ceramic

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@ceramic
Before buying anything give EqualizerAPO a try: if you already have a desktop/tower PC you can use its on board audio sound card/DAC (the 3,5mm jack outputs on the mainboard behind the case; which is probably a 5.1 or 7.1 version) to experiment with it and see if it suits your needs.
I am actually using it to cross my fullrangers with a cheap sub and also using it to equalize the in room response (plus downmix/virtual surround while watching movies/TV shows with my headphones). It's a neat and powerful software and you can use it for every kind of DSP.
I think there are a few discussions about how to use it in the forum, a search should do the trick :)
thank you for the reply.
i am looking for: a digital signal going to the receiver.

i already have a few options to send analog signal to the receiver.


another member mentioned EqualizerAPO, i will take a look at what it has to offer.
 
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ceramic

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Yes. EqAPO cannot copy or synchronize between multiple interfaces so you have to use a single interface that provides all the channels needed.


How will you then have the satellites and subwoofers at the same volume?
thank you, i understand.
so a crossover/eq on the pc, send that over USB to a MULTICHANNEL interface and connect that digitally to the receiver.

i am looking for L + R + Sub, a 2.1 system. the sub's volume will be preset in the crossover, sent to the receiver, the receiver decodes the signals for L, R and (active sub).
 

EDMoser

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if your computer and receiver have HDMI you can send individual channels via HDMI to the receiver and use Equalizer APO to set crossovers for each channel. You have to configure your PC to output the number of channels you require.
 
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ceramic

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if your computer and receiver have HDMI you can send individual channels via HDMI to the receiver and use Equalizer APO to set crossovers for each channel. You have to configure your PC to output the number of channels you require.
thank you for the input.
unfortunately, my favorite receivers don't have HDMI capabilities, but i could use that option for some of the receivers....

i just realized, since this option only requires an HDMI cable, maybe i could use an HDMI capable receiver for digital processing and use the multichannel output (digital or analog) from that receiver to go in some of my favorite receivers that don't have HDMI capabilities.
 
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ceramic

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i just learned that S/PDIF PCM is limited to two channels.
that means the signal between the pc and receiver has to be in the DTS format (the receiver decodes the signal, decides what goes to the R, L and Subwoofer channel).

i installed Equalizer APO. at the first glance the low pass filter needed for the sub is somewhat limited to how steep the slope can be set. (are the digital pass filters also named 1st, 2nd, 3rd, etc order?)
 

zermak

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i installed Equalizer APO. at the first glance the low pass filter needed for the sub is somewhat limited to how steep the slope can be set. (are the digital pass filters also named 1st, 2nd, 3rd, etc order?)
You can sum up (meaning just adding the same in the list) as many filters as you want and even make your own math/scripts to have better or just different slopes.
This is a Low Pass filter I use with a 48db/oct slope (you can change N to have a steeper/relaxed slope and Fc to select your cut-off frequency):
Code:
# LP
Eval: N = 4
Eval: Fc = 100
Eval: Q=1/(2*sin((pi/N)*(0+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
Eval: Q=1/(2*sin((pi/N)*(1+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
Eval: Q=1/(2*sin((pi/N)*(0+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
Eval: Q=1/(2*sin((pi/N)*(1+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
It is not my work, I found it in the EqualizerAPO sourceforge discussion page (source).
 
OP
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ceramic

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You can sum up (meaning just adding the same in the list) as many filters as you want and even make your own math/scripts to have better or just different slopes.
This is a Low Pass filter I use with a 48db/oct slope (you can change N to have a steeper/relaxed slope and Fc to select your cut-off frequency):
Code:
# LP
Eval: N = 4
Eval: Fc = 100
Eval: Q=1/(2*sin((pi/N)*(0+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
Eval: Q=1/(2*sin((pi/N)*(1+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
Eval: Q=1/(2*sin((pi/N)*(0+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
Eval: Q=1/(2*sin((pi/N)*(1+1/2)))
Filter 5: ON LPQ Fc `Fc` Hz Q `Q`
It is not my work, I found it in the EqualizerAPO sourceforge discussion page (source).
thank you very much, i think i'm getting a hold on this software.. i have not tried to 'manually' edit filters but i did manage by luck to stack a few filters and obtain a desirable slope on the LPF.
 

sarumbear

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If I understand you correctly you want to feed a 2.1 system consisting of Left, Right & Sub speakers, using digital signals.

This means you will feed three amplifiers or three active speakers, hence you need three digital signals, Left, Right & Sub. This can be achieved using two SPDIF or an HDMI connections. There is no other digital signal that satisfies your need. (There is ADAT but it’s now a legacy format.)

The L & R signals require a high pass (HP) filter to cut the sub-bass frequencies which will be handled by the subwoofer. The sub signal is the sum of L & R which is low pass (LP) filtered at the same frequency of the HP above. This can be done on a computer using the software mentioned above.

We may help you further if you tell us about your amplifiers and how you will convert from digital to analogue.
 
OP
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ceramic

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If I understand you correctly you want to feed a 2.1 system consisting of Left, Right & Sub speakers, using digital signals.

This means you will feed three amplifiers or three active speakers, hence you need three digital signals, Left, Right & Sub. This can be achieved using two SPDIF or an HDMI connections. There is no other digital signal that satisfies your need. (There is ADAT but it’s now a legacy format.)

The L & R signals require a high pass (HP) filter to cut the sub-bass frequencies which will be handled by the subwoofer. The sub signal is the sum of L & R which is low pass (LP) filtered at the same frequency of the HP above. This can be done on a computer using the software mentioned above.

We may help you further if you tell us about your amplifiers and how you will convert from digital to analogue.
thank you mr sarumbear, that is exactly what i ended up using, an HDMI connection containing L/R and Sub signals going to a receiver. the high and low filters are set in EQ APO along w/ a notch filter where the measurements indicated a room/speaker placement related bump of +2db or so...
 

sarumbear

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thank you mr sarumbear, that is exactly what i ended up using, an HDMI connection containing L/R and Sub signals going to a receiver. the high and low filters are set in EQ APO along w/ a notch filter where the measurements indicated a room/speaker placement related bump of +2db or so...
I am glad to be able to help you.
 
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