Micro Linear Corp. announced the industry's first family of HDTV video filters. The new devices are output reconstruction filters that replace the dozens of discrete components needed to "clean up" video data after it has been converted to analog for playback on a TV.

The high-integration devices will lower cost, reduce board space and improve the reliability of systems such as HDTVs, set-top boxes, DVDs, video cards for personal computers, industrial imaging, etc.

The ML6426 family consists of six devices each optimized for one of the many HDTV transmission formats. Each device contains a DC restore circuit, three 4th-order low-pass filters and three 6dB 75ohm video amplifiers for RGB and YUV video outputs.

Within the HDTV standard there are many different video formats with different degrees of screen resolution. The higher resolution video formats call for high bandwidth output filters. For example, the high-bandwidth ML6426-6 offers a cutoff frequency of 48Mhz which is idea for the highest performance HDTV format that uses 1080 vertical lines, 1920 pixels per line and a clock rate of 162MHz, or 74MHz sample rate.

In addition to supporting high resolution formats, an HDTV system must be designed to handle the low resolution formats and intermediate resolutions formats. The ML6426-1 offers a cutoff frequency of 6.7MHz which is ideal for the lowest performance HDTV (SDTV) format that uses 480 vertical lines, 640 pixels per line and a data rate of 12MHz. The middle three devices in the new family offer cutoff frequencies of 12, 24, 30, and 36MHz for use with the intermediate resolutions.

According to Tony Ochoa, Product Marketing Manager, "Our filters represent an important technology for lowering the cost and improving the reliability of HDTV systems. We have integrated one of the last remaining discrete functions in HDTV. Where there had been an array of discrete components there will now be an integrated circuit. The improved integration also allows the use of multiple devices that enable a system to work with multiple HDTV formats."

Although this is a fairly typo-rich press release there are only a couple of mis-stated facts, such as differential phase being measured in "percent" rather than "degrees." Also the 48 MHz cut-off for the ML6426-6 does not make it suitable for 1080P and the quoted total harmonic distortion is actually double (0.8%) if you take the data sheet as the reference. Rather more annoying here, however, is the continual reference to HDTV instead of DTV (the public is confused enough, let's not start confusing ourselves) and the use both here and in the data sheet of "YUV" instead of "Y, B-Y, R-Y." ("U" and "V" are the weighted color-difference signals for a PAL encoder.)

Despite my moaning, these will be extremely well-received components. The majority of applications will require all six to be used to cope with the plethora of standards that will be transmitted from ATSC's Table 1. At some stage the market will require for some -- if not all -- of the filters to be combined in one part with external switching between frequencies. These are all Butterworth filters designed for minimum overshoots and a flat group delay. They are ac-coupled in and out with the input using a sync-tip clamp needing an external CMOS compatible H-sync; I see no evidence that Micro Linear has provided for the absolutely certain use of tri-level syncs (in the higher standards at least) and the user will have to devise external processing to convert tri-level to a suitable unidirectional H-pulse for the filters.

The parts are for 5-V supplies with a 52-mA current drain and the gains quoted will drive a 35 pF capacitive load; they are in a narrow 16-pin SOIC. Noise at 1.0 mVrms across the board is probably acceptable for all GBR use and most color-difference uses. Crosstalk is quoted at -45 dB at 1 MHz but there are no numbers for any higher frequencies. The absolute group delays at 100 kHz range from 60 ns for the 6.7-MHz filter down to 16 ns for the 48-MHz part. Flatness is extremely good with those two parts showing 7.0 ns at 4.43 MHz and 8.0 ns at 10 MHz, respectively.