Pixel grids, bit rate and compression ratio
Dec 1, 2007 8:07 AM
Audio, video and data distribution can be reduced to one issue — channel capacity. The function of compression is to reduce digitized audio and video to data rates that can be supported by a channel; hence, the less data there is to compress, the easier it is for a compression engine to encode content at an appropriate bit rate for the target transmission technology.
Smaller pixel grids and lower audio sampling rates require lower compression ratios. A lower compression ratio means that less of the original content is discarded during compression, resulting in a reconstruction of the content with the highest quality audio or video that can be delivered over a limited bandwidth channel.
A single 20-bit PCM audio channel sampled at 48kHz has a data rate that is less than 1Mb/s, while SD-SDI is at 270Mb/s and HD-SDI is at 1.5Gb/s. Because video requires a significantly larger amount of data than audio, this tutorial will focus particularly on video.
A numbers game
Video formats vary largely and must be suited for the target delivery channel and reception device. Table 1 is a comparison of total pixels for common pixel grid dimensions. With respect to the three-screen scenario, HD and SD pixel grids are appropriate for broadcast DTV. VGA, CIF and QVGA are suited for broadband Internet delivery, while QVGA and QCIF can be used for mobile video services.
|
Resolution |
Horizontal Pixels |
Vertical Lines |
Total Pixels |
|
HD |
1920 |
1080 |
2073600 |
|
HD |
1280 |
720 |
921600 |
|
SD |
720 |
480 |
345600 |
|
VGA |
640 |
480 |
307200 |
|
CIF |
352 |
258 |
90816 |
|
QVGA |
320 |
240 |
76800 |
|
QCIF |
176 |
144 |
25344 |
Table 1: Picture element comparison for common display grid dimensions.
Frame rate is another factor that influences the amount of compression necessary to fit a given pixel grid into a distribution channel. A 60Hz refresh rate, for example, requires twice as much data throughput as 30Hz and doubles the amount of compression necessary.
Color depth is an additional factor that influences bit rate. MPEG limits luminance and chrominance data to 1B; but in professional applications and when video is delivered over HDMI, luma and chroma data can be words that are 10- or 12-bits long. A 12-bit word has 50 percent more data than an 8-bit word over a given time and increases the amount of compression required.
Channel capacity
Channel capacities for DTV, Internet and cellular service transmission channels vary as well. DTV is defined by the ATSC MPEG-2 transport stream data rate constraint at 19.39Mb/s. For Internet broadband delivery, data rates are 768Kb/s as used by DSL and 1.5 Mb/s, commonly referred to as T-1. For delivery to 3G cell phones, there’s a 2Mb/s maximum data rate indoors and 384Kb/s outdoors.
The following examples will use YUV color space (luminance and color difference signals R-Y, B-Y) and 8-bit color word depth as specified in the MPEG standard.
Table 2 presents the various display formats and their associated data requirements. It is important to note the number of bits per frame in order to understand how widely data rate varies. The difference in the number of pixels from SD to HD display resolutions is an increase of three to six times, and, as the table illustrates, frame refresh rate (along with scan method) can impact bit rate.
Format |
Pixel Grid |
Pixels |
YUV B/frame |
bits/frame |
15Hz Mb/s |
30Hz Mb/s |
60Hz Mb/s |
|
HD |
1920x1080 |
2073600 |
4147200 |
33177600 |
500 |
995(1080i) |
1991(1080p) |
|
HD |
1280x720 |
921600 |
1843200 |
14745600 |
221 |
443 |
885(720p) |
|
SD |
720x480 |
345600 |
691200 |
5529600 |
84 |
166(480i) |
332(480p) |
|
VGA |
640x480 |
307200 |
614400 |
4915200 |
74 |
150 |
300 |
|
CIF |
352x258 |
90816 |
181632 |
1453056 |
22 |
44 |
87 |
|
QVGA |
320x240 |
76800 |
153600 |
1228800 |
18 |
37 |
74 |
|
QCIF |
176x144 |
25344 |
50688 |
405504 |
6 |
12 |
24 |
Table 2: Bit rates for various pixel grids and refresh rates.
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