Power Forces Changes In Portable Audio Design

By Pallab Chatterjee
Power is becoming an overriding issue in the analog world, and nowhere was this more apparent than at the recent NAMM show.

The annual show, which is run by the National Association of Music Merchants,
featured mostly iOS applications and higher-performance hardware plug-ins, although Android development is starting show up. The recent releases of Android support the application-programming interfaces for both mono and stereo audio processing using internal microphones, and also feature USB-based connectivity to external hardware for the audio plugin.

These audio plug-in pieces, including those with the 30pin iPhone/iPad connector, nominally run using the 5v and 3.3v supplies connector rather than the 12v Firewire supply line. The 5v and 3.3v pins support both device charging and higher current operation of connected devices. A key parameter is making the connection with low noise so the device can bring in audio clearly. On the Android and standard USB side the new power supply level is 5v.

For engineers in the digital world, this doesn’t seem like much of a challenge. For the recording and audio industry, it clearly is. The dominant technology for microphones has been an XLR jack (a large format ¾-inch 3-5pin jack) that supports 48v phantom power, or a nominal 12v when used for DMX lighting applications. The new microphones (mono or stereo) for these portable devices include the following blocks: microphone pickup, preamplifier, high-gain amplifier, digitizing subsystem, logic interface and memory for output of the result. Some of the blocks forgo the digitizing block and instead provide line-in connections to the device.

The current baseline is 16bit @ 44KHz, and the “prosumer” systems are moving to 24 bits at 48KHz or 96KHz sampling rates. These systems have to process the signals down to the microvolt levels for USB products. They typically have a single battery operation, and a very short signal path prior to it being digitized. Once digitized, the data is not noise-sensitive and can be transferred digitally without a corruption issue. The low power environment, however, is a challenge for systems that want to use the processing capabilities of the mobile device for the data preparation. These designs need to send a clean audio signal, nominally to the line-in L&R connectors in analog form.

The noise floor and signal integrity in the analog domain is more difficult as the power voltage drops, and a big issue in battery-operated “floating ground” systems. These issues are being addressed by new suppliers in the mobile audio space using standard interface ICs, and by established companies with a history in audio. Companies such as Tascam, Samson, Roland, Blue, Yamaha, and Sony are now being joined by startups that are seeking the digital processing power of the mobile products to join this growing sector.

The power envelope for these devices is bounded by the 480mA peak power of the USB interface, and typically less than 200mA for extended operation. As a result, a new reliance on low-power DSP cores and automated mode control is appearing. These devices are dominated by MHz-class microcontroller cores from ARM, MIPS, Tensillica and Imagination Technologies.

The operating specification for these devices is not peak power, but total power on a mA/hr basis. Most devices require six to eight hours of operation per charge. This total operating power requirement is now pushing the designers to review their modeling and simulation flow to support long-run transient behavior. This behavior has to include battery draw and decay performance as it directly affects the location and depth of the noise floor in the analog signal path. In many cases, the change in battery charge impacts the effective load and impedance of the device, which may alter the analog signals. The systems with the larger DSPs that are placed as close to possible to the data converters near the microphone have so far exhibited the best long term operating performance.