H.261 Datasheet, PDF (30/31 Pages) List of Unclassifed Manufacturers – LINE TRANSMISSION OF NON-TELEPHONE SIGNALS

Several measurements should be made during the sequence length and the average period obtained. Several tests should be
made to ensure that a consistent average figure can be obtained for both encoder and decoder delay times.
Average results should be obtained for each combination of picture format and bit rate within the capability of the particular
codec design.
NOTE – Due to pre- and post-temporal processing it may be necessary to take a mid-level for establishing the transition of the
identification mark at points B and C.
B
Vide o
A
co der
V id eo
de cod er
FIGURE C.1/H.261...[D10] = 6CM
FIGURE C.1/H.261
Measuring points
C
T1 50248 0-90/d10
Annex D
Still image transmission
(This annex forms an integrat part of this Recommendation)
D.1 Introduction
This annex describes the procedure for transmitting still images within the framework of this Recommendation. This
procedure enables an H.261 video coder to transmit still images at four times the normal video resolution by temporarily
stopping the motion video. Administrations may use this optional procedure as a simple and inexpensive method to transmit
still images. However, Recommendation T.81 (JPEG) is preferred when the procedures for using T.81 within audiovisual
systems are standardized.
This procedure can provide high quality image transmission with effects similar to those of progressive and hierarchical
schemes. Minimal changes to H.261 (low cost), backward compatibility with existing terminals, and flexibility in image
quality versus transmission speed were the key considerations in its development.
NOTE – The encoder would set a previously unused bit in PTYPE to “0” when it transmits a still image (unused bits should be
set to “1”). A decoder that ignores this bit would receive the image as normal video. A decoder that goes into an error condition when this
bit is “0” would most likely freeze the previous video frame, and resume when this bit is reset to “1”. A decoder having this new capability
could display the image in a higher resolution, transfer the image to a separate graphics display and hold the image when video resumes,
print and/or save the image, etc.
D.2 Still image format
The still image format is four times the currently transmitted video format. If the video format is QCIF, then the still image is
a CIF frame. If the video format is CIF, which contains 352 × 288 luminance samples, then the still image contains 704 × 576
luminance samples, and a corresponding increase in the number of chrominance samples (a CCIR-601 frame).
Recommendation H.261 (03/93)
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