디지탈오디오를 Black Burst Reference에 동기시키기 위해서 10-field sequence (10-field ID)를 부가적으로 보낼 수 있으며(SMPTE318M), SPG/TG에서 Field Reference를 Enablce 시킨다.
The transition to digital introduces the need to synchronize digital audio signals within the facility. Most professional digital audio systems use a 48 kHz sample rate conforming to the AES/EBU standards. It is important to ensure that digital audio equipment is synchronized together so that clock rate drift does not occur between equipment. If this happens, clicks can occur within the audio because of misalignment in recognizing the data correctly between devices. It is therefore important to provide a digital audio reference to all digital audio equipment. This reference is usually an AES/EBU signal or, in some cases, a 48 kHz word clock. The Tektronix TG700 Audio Generator module AG7 provides AES Silence and word clock outputs, as well as four other AES/EBU outputs, which can provide a range of test tones or silence. The AG7 can also be locked to the video reference for synchronization between audio and video equipment.
For 625/50 line systems, there is a direct relationship between the 25 Hz video and 48 kHz audio of 1920 audio samples per video frame. There are 192 frames within AES/EBU digital audio structure, which produces exactly 10 audio interface frames per video frame. However, for NTSC, because of the frame rate of 29.97 Hz (30/1.001), there is a non-integer number of samples per frame. Therefore, it takes five NTSC frames in order to have an integer number of audio samples – a total of 8008.
To assist in the synchronizing of digital audio to an NTSC black burst reference with a field frequency of 59.94 Hz, an optional ten-field identification sequence can be used as specified in SMPTE 318M. The sequence can also be used in multi-format environments to synchronize equipment operating at 23.976 Hz (24/1.001). For example, 1080 progressive at 23.976 Hz provides a means for a direct transfer of film frames to digital files.
The SMPTE 318 Timing Reference, shown in Figure 11, is inserted on line 15 and 278 of a NTSC 525/59.94 Hz signal. The first pulse (1) is always present at the start of the ten-field identification sequence. Pulses (2-6) are defined in Table 3 and represent the ten-field sequence count. The end pulse (6) is always absent on line 15 and always present on line 278. The TG or SPG signal generator platform provides the ability to genlock to SMPTE 318M with the AGL7 analog genlock module and provides SMPTE 318M output references with the BG7 black burst generator with CB color bar option.
(출처: 20W_18580_0_0.pdf of Tektronix)