Sound

This article is a stub. You can help the OLPC project by expanding it.

A few notes on sound: Ogg vorbis and speex as our preferred codecs for general audio and human speech.

Vorbis, Theora, Speex (and FLAC) are all under the xiph.org umbrella, and all can be sent in .ogg files.

We are also looking into using Pulse Audio Server, which uas transparency across the network.

Speex

Gstreamer has a speex decoder, so things which use gstreamer, like the totem Browse plug-in, should play it.

Speaker Capabilities

(most data from the leader of the TamTam project Ethrop who might have more data on the subject)

The speakers in the XO are from and for cellphone speakers. They are optimized for voice, and have less quality frequency response at the low end of the spectrum. The XO speakers have a severely biased frequency response. We have recently performed a thorough analysis of the audio response curve of the machine and there is a spectacular 12dB peak between 3000 and 4500 Hz, this on all models. I suspect these are mobile phone speakers designed for voice clarity. What this means is kids will likely crank up the volume so that they can hear some of the lower frequencies. Since the physical size of the speakers prohibits any frequencies below 350 HZ, as they try to get a decent bandwidth, they will get the "membrane-against-the-casing" distortion (which has the merit of making the kids lower the volume but risks killing the speakers if done routinely). Someone on the hardware side really should look at the long term prospects for audio hardware failure and see what correction we can bring, by limiting signal output and/or equalising the output of the AD1888 (we dont know what can be done on chip...)

The speakers start rolling off at about 600 Hz and are virtually worthless below 400 Hz.

The hardware has a one-pole highpass filter at about 400 Hz (I forget the exact frequency but it doesn't matter much) in order to reduce the amount of useless LF energy that is presented to the speakers. The rolloff is only in the speaker path; the headphone path has flat response across the audio band.

In my experience, equalization doesn't improve the sound from the speakers very much. They sound tinny and weak no matter what you do. Taming the big peak in the 4 Khz range is of some value, but most program material has little information in that region, so the perceived improvement is small. Boosting the bass makes things worse - the speakers don't have enough air-moving capacity (cone diameter times linear motion range) to render low frequencies, and sending them more signal just slams the mechanical structure against its physical limits, causing distortion and possible damage.

DTMF

For your collective interest, the speakers can reproduce DTMF tones reliably provided the levels are set down from maximum.

At lunch today on a B2 with build-debian, the dtmfdial package was used to transmit tones over a ham radio for making an IRLP request. The DTMF tones include 697 Hz for the top row.

In listening to podcasts, certainly headphones sound better.

Overcoming lack of Tonic

Music activities should thus default to a bassoon.

The odd thing about a bassoon is that the fundamental frequency is nearly absent. The ear-brain system fills in this frequency, making the bassoon sound very low pitched without actually containing much of the very low frequencies.

At the other extreme, a sine wave is worst case. Recorders produce this, and flutes nearly do.

See also

• Hardware specification
• Video
• Game development#Sound
• GStreamer