Digital bass management

[tiroth]

Here is some info on using a Technics SH-AC300 decoder to extract a digital bass signal from PCM or AC-3 bitstreams. The intent would be to use this to obtain a digital bass channel that is the sum of the L+R bass, plus LFE in a multichannel setup. It requires adding a S/PDIF transmitter to the unit.

I got this idea when I noticed that this processor was routing bass to the subwoofer output regardless of main speaker size setting.

This is what the sub output looks like:

[tiroth]

The sub LP filter is 24dB/octave at 100Hz. Fortunately most audiophile applications will probably cross the sub lower than this, so the filter won't interfere too much.

The speaker size setting only affects whether or not a 12dB/octave filter is applied to the main channel outputs; the sub crossover is the same.

[tiroth]

As you can see in the last pic, the digital filter cuts off suddenly above 1.5kHz, so you'd better have another filter taking over at some point.

Before I go on to how I implemented a digital out, here are some gotchas:

• For PCM input, to get the sub output, you need to be in Surround mode. (For AC-3 content you can stay in AC-3 mode)
• The sub output is 16dB down from the input in PCM mode
• None of the level controls operate when you tap the digital out

[tiroth]

To get a digital out, you need to tap these signals off of the decoder daughtercard:

SDIN2 at CS4226 pin 33
LRCK R28
SCK R29
MCK R57
RESET at CS4226 pin 8

Audio is sent to the CS4226 in left-justified mode, 24-bit, 48kHz. MCK is 12.288MHz. This is perfect for CS8405A or similar. I won't post a schematic since the datasheet has all you need.

I found that I needed to add a 10K pulldown to the SCK line in order to get everyting to function. (??)

Now what do you do with this? I'm using it to get a digital line to send to a Behringer unit to do bass EQ. If you do multichannel audio you could also use it to grab the center channel for EQ, but then there is a need to switch the mode. I am considering how to add a circuit that automatically switches the mode depending on the input. Not quite sure how yet but it shouldn't be too tough.

If your goal is an all-digital signal chain, this could help you out. I have seen these processors go as cheaply as $35 on eBay, much less than buying a DCX2496 simply to do the routing. (If you know you want DEQ2496s for EQ)

Pictures to come

[tiroth]

A simple circuit to auto-switch the mode, using the 'AC-3' signal off of daughterboard connector CN02.