Video Module Interface (VMI) for ICs
APPLICATION NOTE
AN9738 Rev. 0.00 Page 1 of 9
Sep 1997
AN9738
Rev. 0.00
Sep 1997
Introduction
VMI was developed in cooperation with several multimedia
IC companies in order to standardize the video interfaces
between devices such as MPEG decoders, NTSC/PAL
decoders, and GUI accelerators. It is primarily based on the
output interface and timing of the Philip's SAA7111
NTSC/PAL decoder.
Video Data Format
An 8-bit 4:2:2 YCbCr interface is normally used, similar to
that used by the BT.656 parallel interface. However, the EAV
and SAV sequences that BT.656 uses are not present.
The 4:2:2 YCbCr data is multiplexed into an 8-bit stream:
Cb
0
Y
0
Cr
0
Y
1
Cb
2
Y
2
Cr
2
, etc. Figures 1 and 2 illustrate the
format for 525/60 and 625/50 video systems, respectively,
using 8-bit YCbCr data.
The stream of active data words always begins with a Cb
sample. In the multiplexed sequence, the co-sited samples
(those that correspond to the same point on the picture) are
grouped as Cb, Y, Cr.
VMI does not define a specific pixel clock rate. However,
most rectangular pixel applications sample each line of video
at 13.5MHz, generating 720 active samples of 24-bit 4:4:4
YCbCr data, as shown in Figures 3 and 4. This is converted
to 16-bit 4:2:2 YCbCr data, resulting in 720 active samples of
Y per line, and 360 active samples each of Cb and Cr per
line. The Y data and the CbCr data are multiplexed, and the
13.5MHz sample clock rate is increased by two to 27MHz.
VMI also does not define any horizontal or vertical blanking
intervals, using instead a programmable blanking signal
(VACTIVE). For most rectangular pixel applications, the ver-
tical blanking intervals will be as shown in Figures 5 and 6.
Note that active resolutions other than 720 x 486 and 720 x
576 may be supported (effectively cropping the image) by
adjusting the timing of VACTIVE.
Square Pixel Variation
A variation using square pixels may also be used. Instead of
a 27MHz clock, a 24.54MHz clock is used for 525/60 video
systems (640 x 480 active resolution), and a 29.5MHz clock is
used for 625/50 video systems (768 x 576 active resolution).
FIGURE 1. TYPICAL VMI 8-BIT DATA FORMAT FOR RECTANGULAR PIXEL 525/60 VIDEO SYSTEMS
CO-SITED
START OF DIGITAL ACTIVE LINE
NEXT LINE
VACTIVE CONTROL SIGNAL
CO-SITED
BLANKING
1716
1440
DIGITAL
VIDEO
STREAM
8
0
1
0
8
0
1
0
8
0
1
0
8
0
1
0
8
0
1
0
8
0
1
0
8
0
1
0
C
B
YC
R
YC
B
YC
R
Y8
0
C
R
Y