找回密碼
 立即註冊
搜索
樓主: VR2GY

CQ游 慶回歸25年VR25RC戶外電台

[複製鏈接]
 樓主| 發表於 29-7-2022 12:56:21 | 顯示全部樓層
1
sstv-11.jpg
 樓主| 發表於 29-7-2022 12:57:00 | 顯示全部樓層
1
上 sstv.jpg
 樓主| 發表於 29-7-2022 12:58:38 | 顯示全部樓層
1
0001.JPG
 樓主| 發表於 29-7-2022 21:20:36 | 顯示全部樓層
1慢扫描电视(Slow-scan television)是业余无线电爱好者的一种主要图片传输方法,慢扫描电视通过无线电传输和接收单色或彩色静态图片。

慢扫描电视的一个术语名称是窄带电视。广播电视需要6MHz的带宽,因为帧速为25到30fps。慢扫描电视的带宽只有3kHz,因此慢扫描电视是一种慢得多的静态图像传输方法,通常每帧需要持续8秒或若干分钟。

 樓主| 發表於 29-7-2022 21:21:54 | 顯示全部樓層
业余无线电操作员通常在短波(或高频)、甚高频、超高频波段使用慢扫描电视。


概念
慢扫描电视的概念由考普瑟·麦克唐纳提出,在1957年到1958年他发展了第一个使用摄像管和电子监视器的慢扫描电视系统,该系统使用120行,120列的分辨率在3kHz电话信道中传输黑白静态图片和音频信号。

 樓主| 發表於 29-7-2022 21:56:15 | 顯示全部樓層
在航天领域的早期应用
宇航员戈尔登·库勃在水星9号中的慢扫描电视广播


慢扫描电视曾用于 月球3号对月球另一面图片的传输








东方一号是最早使用慢扫描电视的航空器,其慢扫描电视系统Seliger-Tral-D由早期可视电话改进,Seliger-Tral-D带有两部使用LI-23光电管的摄像机,可传输10fps的100线视频图像。


 樓主| 發表於 30-7-2022 18:58:31 | 顯示全部樓層


SSTV mode information       


The use of frequency modulation to convey the picture information makes analogue SSTV very immune to impulse interference. Although co-channel amplitude interference will degrade an SSTV picture the FM capture effect means that a strong SSTV signal will tend to override weaker signals.


The image on the right shows a typical spectrum display for an SSTV transmission.


SSTV spectrumThe left side of the display represents an audio frequency of 1,000 Hz and the right hand side represents 2,400 Hz. The three vertical lines are at 1,200 Hz, 1,500 Hz and 2,300 Hz.


Picture Content


The image information is transmitted between 1,500 Hz and 2,300 Hz.  Black corresponds to a frequency of 1,500 Hz and white corresponds to 2,300 Hz.  The 800 Hz band in between represents a continuous gray scale between pure black and pure white, so that 1,900 Hz represents mid gray.


The number of gray levels which can be transmitted is limited by the ability of the receiving software to discriminate between one frequency and another, but in practice it is possible to display a continuous spectrum between black and white without visible steps.


It should be noted that since only a single audio sub-carrier is used the transmission will only contain a single frequency at any instant.  The above display represents the information gathered over a short period, which includes a segment of image information and at least one sync pulse.  This particular image segment contains some pure black picture content ranging through to mid gray, but there is no pure white content.


Line Sync Pulses


Line synchronization pulses (sync pulses) are transmitted at a lower frequency than the image information, normally at the start of each line so that the start of the line can be clearly identified. They are visible in the spectral display as a peak around 1,200 Hz. (The small peak around 1,350 Hz is probably noise.)


The main function of the sync pulses is to allow an image to be properly synchronized even if the start of the transmission was missed.  Once horizontal synchronization has been established the sync pulses can generally be ignored provided the timing of the receiving equipment is accurately aligned to the transmitting equipment.  This has the advantage that noise and interference will not affect picture synchronization, but if there is any timing discrepancy the picture will be received with a slant.  For this reason, some programs provide an option to make use of the sync pulses to re-synchronize the lines as the picture comes in.  Although this may correct any slant, results can be unreliable under less than perfect conditions.


If the image is sent by SSB and is not tuned in correctly this will cause the spectrum to shift to the right or the left. The signal is correctly tuned when the sync pulses fall at 1,200 Hz.


Decoding Considerations


At any one time during picture transmission the frequency of the SSTV sub-carrier determines the gray level of the image. Thus, picture quality depends on the ability of the receiving equipment to accurately measure the instantaneous frequency of the sub-carrier. For most practical purposes one half cycle is the theoretical smallest quantum of information which can be sent using such a system.


At black level: f = 1,500 Hz, so cycle time t = 666 uS and t/2 = 333 uS


At white level: f = 2,300 Hz, so cycle time t = 434 uS and t/2 = 217 uS


Waveform symmetryThere is therefore little point in attempting to transmit picture information with a pixel time significantly less than 217 uS.


Although digital signal processing techniques have been employed to try and accurately measure the frequency of the received waveform within less than half a cycle with some degree of success, the tried and tested method is to measure the time intervals between the points at which the waveform passes through the zero axis. This has proved to be a simple, accurate and robust technique, although it is subject to one major pitfall.


Consider a constant audio frequency as produced by a uniform gray image. If the signal input circuits are less than perfect they can easily produce an asymmetrical waveform, shifting the waveform above or below the zero axis. This will cause adjacent half cycles to be alternately longer and shorter, which is interpreted as two different shades of gray. Depending on the number of half cycles which go to make up each pixel, this can result in noticeable patterning on the image.


The accepted solution is to average the last two half cycles so that variations due to waveform asymmetry are smoothed out. This effectively eliminates such patterning problems, albeit at the expense of image resolution. As a consequence, the practical limits of resolution are generally closer to the full cycle period rather than the theoretical half cycle limit.  Therefore, pixel times of less than about 400 uS cannot be achieved without degrading picture sharpness, and significantly greater times are required to obtain true pixel-for-pixel reproduction.  


20 January 2008




 樓主| 發表於 30-7-2022 19:06:21 | 顯示全部樓層
1
Polish_20220730_190511033.png
您需要登錄後才可以回帖 登錄 | 立即註冊

本版積分規則

中華業餘無線電研究會 - 業餘無線電考試(筆試)班

手機版|小黑屋|存檔|VR2GY 網站

GMT+8, 29-3-2024 21:45 , Processed in 0.036994 second(s), 18 queries , Gzip On.

本討論區基於 Discuz! X3.5

版權所有 © 2005-2024 VR2GY.com,保留一切權利


重要聲明:本討論區是以即時上載留言的方式運作,VR2GY.com 對所有留言的真實性、完整性及立場等,不負任何法律責任。而一切留言之言論只代表留言者個人意見,並非本網站之立場,讀者及用戶不應信賴內容,並應自行判斷內容之真實性。於有關情形下,讀者及用戶應尋求專業意見(如涉及醫療、法律或投資等問題)。 由於本討論區受到「即時上載留言」運作方式所規限,故不能完全監察所有留言,若讀者及用戶發現有留言出現問題,請聯絡我們。VR2GY.com 有權刪除任何留言及拒絕任何人士上載留言(刪除前或不會作事先警告及通知),同時亦有不刪除留言的權利,如有任何爭議,管理員擁有最終的詮釋權。用戶切勿撰寫粗言穢語、誹謗、渲染色情暴力或人身攻擊的言論,敬請自律。本網站保留一切法律權利。

快速回復 返回頂部 返回列表