Satellite Radio - At the heart of a Sirius receiver is a custom ASIC chip called the baseband processor, currently the STA240, which is produced by STMicroelectronics. The chip contains embedded ARM7TDMI and ARM946E-S microprocessors synthesized from IP cores. The ARM7 handles decryption keys and subscription mechanisms, while the ARM9 handles audio decompression and control interface functions. The firmware uses eCos for its operating system. Every baseband processor has a unique serial number (or Sirius ID), burned into it at the factory, which is an essential part of the subscription mechanism. Another major section of a Sirius receiver is its tuner. The tuner is also comprised of a custom ASIC, one of which is the STA210. The tuner connects to the antenna, and receives the incoming satellite and terrestrial signals at 2.3 GHz and downconverts them to intermediate frequency signals at around 75 MHz. The IF signals are fed to the STA240, which are digitized, demodulated, de-interleaved, and decrypted using specialized circuits on the chip. The baseband processor utilizes a 16 MB SDRAM memory to buffer four seconds of one of the satellite signals in order to bring it into time coincidence with the other for Maximal-ratio combining. On newer receivers that let you 'pause' live radio, a dual-port PSRAM is employed to store up to 44 minutes of the selected channel. The baseband processor outputs digital audio over an I²S interface, which is fed to a D/A converter to produce the analog audio signal. The front-end of a Sirius receiver is called the head unit, required to display text and provide controls to the user. This is implemented by the third-party designers of Sirius-ready receivers, using a microprocessor of their choice. The head unit and baseband processor communicate over an RS-232 interface, speaking the Sirius Standard Protocol.