James (JJ) Johnston at PNW Section Meeting on portable music player technologies, Oct 20, 2008.
The PNW Section's October 2008 meeting featured James (JJ) Johnston presenting a general overview of how portable music players work, such as the iPod or Zune.
Attendance was approximately 15 members & 9 non-members. Opus 4 Studios of Bothell WA hosted the event.
James (JJ) Johnston, of Neural Audio (formerly Microsoft & Bell Labs) discussed the elements that all portable music players need to work, without discussing proprietary brand specifics.
JJ discussed how a player needs a storage medium, a control system (with an OS, file system, sginal processor & user interface), a headphone drive system and a power scheme, all while remaining small and running as long as possible on a charge.These are powerful little computers. The careful power budget explains the rudimentary DSP available, such as a few fixed EQ profiles.
Some discussion of lossless and perceptual codecs followed, such as bit rate vs. quality. There are some messy legal issues intertwined with many lossy codecs, so progress is a problem.
JJ admonished the group to never use more than one layer of perceptual coding (ie: don't convert from one to another), or it will sound awful. Some of his other tips:
Level control is often a digital PCM operation, which can mean a poor S/N ratio at low levels. Set the output as high as possible without clipping if using the device as a line-out.
Many amp designs usually sacrifice some audio quality for power economy, and the output capacitors required are undersized, affecting low-frequency response. Running into a higher impedance load, such as a line input, helps.
A break was held, and door prizes awarded.
JJ's post-break bonus session was a recap of his participation at the recent AES Convention, a panel recalling the "worst mistakes in audio." JJ's 10 nominations were all accompanied by spirited discussions. For his nominations, and a more detailed meeting recap, see the PNW Section website, http://www.aes.org/sections/pnw/pnwrecaps/
