Re: apis for low latency audio streaming over wifi to ipod touch?
Re: apis for low latency audio streaming over wifi to ipod touch?
- Subject: Re: apis for low latency audio streaming over wifi to ipod touch?
- From: "Ben Allison" <email@hidden>
- Date: Sun, 7 Mar 2010 13:04:13 -0500 (EST)
- Importance: Normal
Ross -
Not sure from your post if the audio is truly 'real time' or not, because
you mention in the last paragraph that the audio tracks are stored
somewhere. If it is truly real-time (that is, you can't buffer/read
ahead) then your biggest problem is going to be codec latency, unless you
are able to stream uncompressed PCM data. Apple does have an AAC LD
codec, but I can't remember if a decoder is available on the iPod touch or
not. Normal AAC (AAC LC) is going to have an unacceptable long latency
with real-time input - about 70ms - because that is now much data the
encoder requires (2 input frames; 1024 samples each) before it can produce
a single encoded frame, and the decoder requires 2 frames before it can
produce any output. End-to-end, you need 3 input frames before producing
your first decoded output frame, and we're not even discussing network
latency. AAC LD is about 25ms end-to-end and AAC ELD can get under 20ms.
Even lossless audio schemes like FLAC or ALAC are going to require too
many input samples to form a frame to have adequate real-time
capabilities.
If the wireless bandwidth is good enough to trust uncompressed audio,
you'll definitely want that. And if you do in-fact have the luxury of
buffering ahead with the input audio - that will help immensely because
you can use a compressed codec.
-Ben
> Hi All
>
> I've been aksed to implement a system where ipod-touch devices are used to
> tune in to real-time audio streams (at the momemtn, the audio from one of
> 20
> DVD->plasma displays in a gallery context but it could be any real-time
> audio).
>
> Can anyone tell me how much of this can be achieved out of the box with
> Apple APIs? Is it possible to get low latency audio streaming over WiFi (I
> guess I need < 20ms) with the right QoS routers etc? What kind of
> buffering/latency should I expect? I'm imagining I'll implement a low
> latency multichannel capture/compress app on a mac server, or perhaps
> there
> is already a product that does this...
>
> The alternative approach is to store the audio on the ipods and just
> transmit timecode accross WiFi. I'm confident I can make that work, but it
> requires video playback devices which output timecode.
>
> Any and call comments appreciated
>
> Thanks in advance
>
> Ross.
>
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