No. S/PDIF carries only compressed digital audio, although it sometimes uses analog connectors.
No, you cannot directly convert a 3.5mm jack (analog audio) to SPDIF (digital audio) using a simple adapter, as they are fundamentally different types of signals. However, you can use a digital audio converter that takes an analog signal from a 3.5mm jack and converts it to a digital SPDIF signal. This typically involves more than just an adapter, as it requires a digital-to-analog converter (DAC) or an audio interface with the appropriate conversion capabilities.
SPDIF is a digital audio interface and is designed to transfer audio from one device to another without converting the signal to analog. Eventually, the signal will become analog as the speakers require an analog signal. to work. Almost all audio devices that have a SPDIF output also have an analog output so rather than buying a SPDIF to analog converter, use an existing analog output. If the device with the digital output doesn't have an analog output, then you don't have much choice about buying a converter unit. The converter is an active device so don't expect to find a simple converter cable. They don't cost a lot and they can be obtained from many specialist electronics stores. Good starting points are Radio Shack in the US or Maplin in the UK
SPDIF can carry stereo (2 channels) of audio.
Toslink and SPDIF (Sony/Philips Digital Interface) are related but not the same. SPDIF is a digital audio interface standard that can transmit audio signals in either electrical (coaxial) or optical (Toslink) formats. Toslink specifically refers to the optical version of SPDIF, which uses fiber optic cables to transmit audio signals as light. Therefore, while all Toslink connections are SPDIF, not all SPDIF connections are Toslink.
You would have to purchase a Digital to Analog converter and connect the SPDIF (orange RCA connector) to the D/A converter's digital input via a RCA/SPDIF cable. Then you would have to connect some type of headphone amplifier to the line outputs of the D/A converter.
No, a S/PDIF is, by definition, a Sony / Phillips Digital Interconnect Format connector, and is always a digital signal. It is usually connected by an RCA cable, and can be carried by cable specified for analog use, but digital cable is better.
Audio and video -- and there are several options within each. Audio is typically digital (COAX or SPDIF) but could also be analog (dual RCA). Video is normally either component or HDMI.
TV audio can be routed to a soundcard either via analog RCA, digital COAX, or SPDIF, depending on what is available on your specific TV and soundcard models.
It's not straightforward to convert between SPDIF and a stereo pair of RCA plugs. SPDIF is a digital standard (SPDIF is an acronym for Sony Philips Digital Interface) and a pair of RCA plugs is analogue. It is possible to buy adaptors that convert from one to the other; if you search for SPDIF to analogue (or analog) convertor with google, you'll find a selection.
A SPDIF interface is a connection that is found on a variety of electronic audio devices, from computer sound cards, minidisk players, home theatre surround amplifiers and many more. SPDIF is short for Sony Phillips Digital InterFace and it is used to transmit audio digitally from one device to another. The digital audio formats supported by this interface are many and include standard 2 channel audio up to 5.1 Dolby Digital channels at high sample rates.
SPDIF is a digital audio protocol developed by Sony and Philips. In order to connect the signal to an anologue input, a digital to analogue converter is required. If the source also has analogue outputs, it is much easier to use them - the digital to analogue converter is built into the source and saves the need for an external one. It might be argued that a good quality external converter can produce a better quality analogue signal than an integrated converter in a DVD player for example. A listening test will help to decide if that is the case.
The optical input on a TV is used for transmission of digital audio, in SPDIF format between devices, only.