Preferred Alignment for Dolby E in HD Systems

System designers and equipment manufacturers may have questions on how to eliminate the ambiguity of Dolby® E alignment measurements with reference to various video formats. To ensure error-free editing and switching of Dolby E signals, we have developed a recommended approach to alignment issues.

Dolby E streams include audio and metadata encoded into blocks of SMPTE 337M data (initially placed on a discrete AES/EBU carrier, or subsequently embedded into SDI). These data blocks are output at a rate associated with a chosen video frame rate, as the Dolby E bitstream is normally carried along with video. To allow error-free cut-style editing and video switching, a small section of null data (or silent samples) exists around the video switch point (also referred to as the “guard band”). The guard-band location is designed to allow a Dolby E bitstream to be delayed, up to a point, as a safeguard against frame errors that may occur in a bitstream upon switching or editing. The measurement of where the guard band ends and the SMPTE 337M Dolby E data burst starts is often called the “line position.” This relates to a specific video line to which the start of the Dolby E data burst is aligned.

The best placement of the Dolby E line position is relatively simple to deduce when there is a limited number of video standards to which the signal must adhere. For example, if standard-definition video is the only format to be used, then determining the preferred line position is straightforward. The placement becomes more complicated when multiple formats come into play, or when there is cross-conversion taking place. For example, if the video format was originally standard definition, and was then upscaled to high definition, the line position measurement of Dolby E against the SD video format would not correctly reflect the Dolby E alignment against the HD video format.

In such cases, the preferred method for determining the alignment of Dolby E with respect to video is to measure the time elapsed from the SMPTE RP168 reference point (the point in time indicating the start of the video frame). According to the SMPTE standard, this reference point is further defined as the point in time where line 1 for 720p 59.94, line 1 for 1080i 59.94, and line 4 for 480 29.97 fps (525 NTSC 29.97 fps) signals align, as well as the point in time where line 1 for 720p 50, line 1 for 1080i 50, and line 1 for 576 25 fps (625 PAL 25 fps) signals align.

In general terms, Dolby E content is in a safe position if the start of the SMPTE 337M data burst occurs after the switch point and the end of the SMPTE 337M data burst occurs before the start of the next video frame.

The following chart provides the recommended alignment locations for Dolby E frames with respect to SD video lines, HD video lines, and the RP168 reference point. It also shows the limits of the delay that can be applied to a Dolby E frame, beyond which the Dolby E bitstream will likely be corrupt upon switching or editing.

Recommended Dolby E Line Positions

		625 25 PAL SD	1080i 50 PAL HD	720p 50** PAL HD		525 29.97 NTSC SD	1080i 59.94 NTSC HD	720p 59.94** NTSC HD
Potentially earliest valid Dolby E position
TV line		8	13	17		12	18	23
µs*		450	450	450		510	510	510
Ideal Dolby E line position –80 µs
TV line		11	19	25		13	21	28
µs*		650	650	650		610	610	610
Ideal Dolby E line position ±80 µs
TV line ±80 µs		12	21	28		14	24	32
µs ±80 µs*		730	730	730		690	690	690
Ideal Dolby E line position +80 µs
TV line		13	23	31		16	26	35
µs*		810	810	810		770	770	770
Potentially latest valid Dolby E position
TV line		30	53	70		26	48	63
µs*		1,860	1,860	1,860		1,400	1,400	1,400
* In relation to the SMPTE RP168 reference point and approximate values.
** In relation to the first (odd) frame.