Booting with s-video configured, but only shows color bars

I got Angstrom booting on my BB-xM with DVI output, but I have run
into a problem in attempting to change the output to s-video (for
"Myvu Crystal personal media viewer" video glasses, which accept RCA
composite video).

As per

I created a new .txt file called boot.cmd in the boot partition of my
SD card that runs Angstrom and put this text in it

setenv bootargs 'console=tty0 console=ttyS2,115200n8 root=/dev/
mmcblk0p2 rootwait ro vram=12M omapfb.mode=tv:ntsc omapdss.def_disp=tv
fixrtc buddy=unknown'

then I opened up a terminal in the boot partition of the SD card and

mkimage -A arm -O linux -T script -C none -a 0 -e 0 -n 'Execute
uImage' -d boot.cmd boot.scr

in order to make it into an executable boot.scr.

When I power up my board with this script in my boot partition, all of
the LEDs come on, I get no DVI output, and it displays color test bars
on my video glasses, but it never shows anything else, (at least not
for 10 minutes or so, which is as long as I have tried)

I also tried putting

run loaduimage
run mmcboot

at the end of the boot.cmd file instead of just "boot", like in the
scripts in the u-boot-scripts, but this does the same thing.

My SD card boot partition has these files: boot.cmd, boot.scr, MLO, u-
boot.bin, uEnv.txt, and uImage. My uEnv.txt file contains

loaduimage=fatload mmc ${mmcdev} ${loadaddr} ${bootenv}
mmcboot=echo Running boot.scr script from mmc ...; source ${loadaddr}

If I delete uEnv.txt, Angstrom boots normally, with DVI output. I
really don't understand what uEnv.txt is doing...

So I suppose my question is, what should the text in my boot.cmd file
be in order to properly boot with s-video as the output, and not just
display color bars forever? Or maybe my uEnv.txt file is not right? Or
does the problem perhaps lie elsewhere? Thank you very much for any