D-3 is an uncompressed composite digital video videocassette format invented at NHK, and introduced commercially by Panasonic in 1991 to compete with Ampex's D-2. It uses half-inch metal particle tape at 83.88 mm/s (compare to D-2's 19 mm and 131.7 mm/s). Like D-2, the composite video signal is sampled at four times the color subcarrier frequency, with eight bits per sample. Four channels of 48 kHz 16-20 bit PCM audio, and other ancillary data, are inserted during the vertical blanking interval. The D-5 format, introduced in 1993 by Panasonic and presently (2006) marketed as D-5 HD, uses the D-3 transport and tape running at roughly double D-3 speed.