Why Mosaic Effect happens when playing MPEG-1 file with Windows Media Player

Question

Hi, I am writing a program to convert video files into MPEG-1 format. You see, there are a set of parameters to define the converting process, like frame rate, bit rate, etc. For MPEG-1 format, only 23.976, 24, 25, 29.97, 30, 59.94, 60 fps(frame per second) are available.
To test the converted MPEG-1 file, I first use Windows Media Player (9.0)  to play the converted file. I found that the MPEG-1 files with frame rate other than 60fps are played well. But when playing the MPEG-1 files with frame rate of 60fps, mosaic effect appears apparently. Then I played these converted files by other decoders which are Moonlight-Elecard MPEG2 Video Decoder, Nero Video Decoder, Libavcodec, and Libmpeg2. With these decoders, the MPEG-1 files with frame rate of 60fps play well with no mosaic effect at all.

I have also tried other encoder software (Panasonic MPEG-1 Encoder) to generate MPEG-1 files. Results are the same: by Windows Media Player, mosaic effect happens only if playing 60 fps MPEG-1 files; other frame rates are OK. And by other video decoders listed above, all frame rates (include 60fps) are OK with no mosaic effect at all.

I use DirectShow to find the decoder used by Windows Media Player, that is Microsoft MPEG1 Video Decoder 2.0. And I found that decoder depends on quartz.dll.

In addition, I convert the file format into MPEG-1 by ffmpeg, the source code is listed below.

int PPMtoMPEG1(const char *input_file, 
                const char *output_file,
                unsigned int width /* = 0 */, 
                unsigned int height /* = 0 */, 
                unsigned int bit_rate /* = 12000 */,
                int overwrite /* = 1 */,
                void *para /* = NULL */)
{
    const int max_int_oct_bits = 20;

    int argc = 14;

    if (width > 0 && height > 0) {        // '-s size' option is needed
        argc += 2;
    }

    if (overwrite) {                    // '-y' option is needed
        ++ argc;
    }

    // We need an additional argument, see below.
    char **argv = new char *[argc + 1];

    // see whether output file exists.
    FILE *fp = fopen(output_file, "rb");
    if (NULL != fp) {
        if (!overwrite) {        // file exists but we are not allowed to overwrite it
            sprintf(error_message, "Output file '%s' already exists\n", output_file);
            return 1;
        }
        fclose(fp);
    }

    int opt_index = 0;

    // dumb argument
    argv[opt_index ++] = "ffmpeg";

    // '-r fps'            set frame rate (default = 25)
    argv[opt_index ++] = "-r";

    char opt_fps[11];
    sprintf(opt_fps, "%d/%d", frame_rate_tab[frame_rate_index], MPEG1_FRAME_RATE_BASE);
    argv[opt_index ++] = opt_fps;

    // '-i filename'    input file name
    argv[opt_index ++] = "-i";

    char *input = new char[strlen(input_file) + 1];
    strcpy(input, input_file);
    argv[opt_index ++] = input;

    // '-s size'         set frame size (WxH)
    if (width > 0 && height > 0) {
        argv[opt_index ++] = "-s";

        char opt_size[max_int_oct_bits * 2 + 1];
        sprintf(opt_size, "%dx%d", width, height);
        argv[opt_index ++] = opt_size;
    }

    // '-b bitrate'     set video bitrate (in kbit/s)
    argv[opt_index ++] = "-b";

    char opt_bitrate[max_int_oct_bits + 1];
    sprintf(opt_bitrate, "%d", bit_rate);
    argv[opt_index ++] = opt_bitrate;

    // '-y'             overwrite output file
    if (overwrite) {
        argv[opt_index ++] = "-y";
    }

    // '-qscale q'         use fixed video quantiser scale (VBR), always 1 to indicate the excellent quality
    argv[opt_index ++] = "-qscale";
    argv[opt_index ++] = "1";

    // '-vcodec codec'     force video codec to mpeg1video.
    argv[opt_index ++] = "-vcodec";
    argv[opt_index ++] = "mpeg1video";

    // '-r fps'            set frame rate (default = 25)
    argv[opt_index ++] = "-r";
    argv[opt_index ++] = opt_fps;

    // output file name
    char *output = new char[strlen(output_file) + 1];
    strcpy(output, output_file);
    argv[opt_index ++] = output;

    // the standard requires that argv[argc] be a null pointer    - from K&R. Section.5.10.
    argv[opt_index] = NULL;

    FFMPEG_PROGRESS_CALLBACK cb = NULL;
    if (progress_callback) {
        cb = ToMPEG1CallBack;
    }

    int ret = ffmpegmain(argc, argv, error_message, cb, para);

    delete []output;
    delete []input;
    delete []argv;

    return ret;
}

Because the users of my program will play the converted MPEG-1 files by Windows Media Player, I have to make sure why mosaic effect happens: is it because Windows Media Player could not play 60fps MPEG-1 files? Or is it because of codes errors? Could you help me? Thanks a lot.