PDA

View Full Version : High-quality headphone audio solution for a Propeller app?



Lord Steve
12-13-2009, 03:32 AM
Currently I am planning on using a Microchip MCP4922 DAC to sit between the Prop and the audio amp stage.· However, the amplifier IC I wanted to use has gone into the "Last Time To Order" phase.· All I need is high-quality stereo·headphone output.· Is there a well-known beloved amp out there that will run from a single 3.3V supply?· Do I really need the DAC for high-quality?· What are some high-quality alternatives?

Thoughts are appreciated.· I'm·not an analog kinda guy.

Phil Pilgrim (PhiPi)
12-13-2009, 04:45 AM
The stereo audio quality emanating from the Propeller Demo Board is really pretty good, and it doesn't use a DAC. You might consult the Demo Board schematic for part names and component values.

-Phil

Lord Steve
12-13-2009, 06:22 AM
That was a good tip.· Thank you.· That amplifier IC only comes in a "space-saving", impossible-for-Steve-to-hand-solder package.· Can anyone recommend another similar IC which comes in a little bigger SMD package (or one with leads)?

Is there a thread or documentation about how the DAC part of the audio path works?

Lord Steve
12-13-2009, 06:51 AM
Is it just using PWM?

Phil Pilgrim (PhiPi)
12-13-2009, 07:02 AM
It uses DUTY mode output from the counters, which is similar to PWM, but not the same thing.

-Phil

lonesock
12-13-2009, 07:35 AM
You may want to check out the MAX4410: www.maxim-ic.com/quick_view2.cfm/qv_pk/3459 (http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3459), it comes in a 14-pin TSSOP package.

Jonathan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
lonesock
Piranha are people too.

Lord Steve
12-13-2009, 08:10 AM
Hey, lonesock.· Thanks for that tip, too.· It's even in production.· :D· Now to wrap my head around the differential duty cycle stuff.

Is the DIFFERENTIAL submode of the DUTY counter mode used in Chip McGracey's microphone_to_headphone.spin solely to provide TWO audio outputs (cuz the electret mic on the Demo Board is, after all, monaural)?· If there is an already-written explanation about this, please point me to it.· This just looks like PWM to me, but it's some different DUTY thing...idk.

Ariba
12-13-2009, 11:50 AM
DUTY mode is PWM, but not in the usual form.

Normal PWM has a constant frequency and only the High-Low ratio changes.

At DUTY mode, the shorter Pulswith (High or Low) is always 1 clock cycle, and the other state (Low or High) is extended,
so that you get the right pulswith. That is the frequency changes with the pulswith.
On 50%:50% you have the highest frequency (40 MHz at 80 MHz system clock!). At 10%:90% you have a frequency of 8 MHz
and at a 16 bit resolution the end values +$7FFF and -$7FFF have a frequency of 4.15 kHz.

This makes this mode ideal for Audio, because quiet signals are around 50% with high frequencies, and loud signals cover the lower PWM frequency.

But there are also downsides: A DUTY output produces a lot of RF noise in the air and on the print.
And High-Quality is very relative. If you have single notes and low signals you will hear some digital noise. If you play music this is normaly not noticable.

Andy

Lord Steve
12-13-2009, 12:15 PM
I figured it out!· Thanks for your pointers.· You saved me ~3$ off my BOM.· :D

Those counters are too cool.· I have attached a .spin program (based off of Chip's microphone_to_headphones.spin) which samples the propeller's built-in sine tables at different angular steps to generate two samples.· These are added together and given to a DUTY mode counter's FRQB at a rate of 44100 Hz.· In my opinion, the playback is high quality.