Matt, There are a few flaws in your statements. You are correct that human hearing is up to 20 kHz, and again with the 40 kHz sampling (this is why CD is 44.1 kHz, to be able to fully obtain a 40 kHz sample with non-ideal filters). However, we are talking about Secure Phones. Phones have a dynamic range of 3.6 kHz, with a sampling of 8000 samples/sec! Also, the phone uses 8-bit mu-law data, for the 64000 bits/sec that you commonly hear. So, without compression, you need a 64 Kbps link to transmit phone-quality audio. The problem with a Secure Phone is getting the data compression to reduce the data to below 14.4 Kbps, in order to use a 14.4 Kbps modem. You need a real-time compression to do this, which is where the DSP chips come in! The encryption isn't a problem, as most encryption algorithms can easily do 64 Kbps! -derek Derek Atkins, SB '93 MIT EE, G MIT Media Laboratory Secretary, MIT Student Information Processing Board (SIPB) PGP key available from pgp-public-keys@pgp.mit.edu warlord@MIT.EDU PP-ASEL N1NWH