by outgoing.mit.edu (8.13.8/8.12.4) with ESMTP id w2EJvsmv004135 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NOT) for osmocom-sdr@lists.osmocom.org; Wed, 14 Mar 2018 15:57:55 -0400 To: osmocom-sdr@lists.osmocom.org From: Theodric Young theodric@mit.edu Subject: Meaning of rtl_fm output values Message-ID: d96cc71b-6d81-d786-ccd1-708c84eba0ff@mit.edu Date: Wed, 14 Mar 2018 15:57:55 -0400 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFjrPIsWRmVeSWpSXmKPExsUixCmqrHusbmWUwc1tLBb7Jl9hcWD0WHXa I4AxissmJTUnsyy1SN8ugSvjx87tbAX7OSv2bJ/G3sC4lr2LkZNDQsBEYvqcdSxdjFwcQgKL mSQWnj/DCuFcZZSY37eNDcKZxizRv/4YI0iLiICixOTf24FaODjYBNQlJv6zAQkLC2hIrL91 jwXE5hWwkvh96Q0TiM0ioCrxdtsWZhBbVCBGomXJB0aIGkGJkzOfgNUzC5hJzNv8kBnClpfY /nYO8wRG3llIymYhKZuFpGwBI/MqRtmU3Crd3MTMnOLUZN3i5MS8vNQiXRO93MwSvdSU0k2M 4FCS5N/BOKfB+xCjAAejEg+vgdrKKCHWxLLiytxDjJIcTEqivPunrIgS4kvKT6nMSCzOiC8q zUktPsQowcGsJMJ7vxConDclsbIqtSgfJiXNwaIkzutuoh0lJJCeWJKanZpakFoEk5Xh4FCS 4D1cC9QoWJSanlqRlplTgpBm4uAEGc4DNFyuDmR4cUFibnFmOkT+FKMxx5+HL9uYOW68eN3G LMSSl5+XKiXO+x5knABIaUZpHtw0UDq4yHbi3CtGcaDnhHkFQAbyAFMJ3LxXQKuYgFZlblsB sqokESEl1cAYvC/tgHRlS0bd5jnmP/RrNsU/5J6oKV7p8ixCsX/y/3f1+69NufA6bvK/ZD3H 9ONOL50fx7tsVejkM1b7PVkywsPsRnRig7Vbm3R4CnNom/9rs3aJA/Oas4WZ3H7tupb8N8np Qy/bcu5Nib3zr84VOPDxw2mnkitF08Nv2O89u0UxJ+n8wa9KLMUZiYZazEXFiQCH7MfT4gIA AA== X-BeenThere: osmocom-sdr@lists.osmocom.org X-Mailman-Version: 2.1.26 Precedence: list List-Id: "Discussion on Osmocom SDR projects (OsmoSDR, rtl-sdr)" <osmocom-sdr.lists.osmocom.org> List-Unsubscribe: https://lists.osmocom.org/mailman/options/osmocom-sdr, mailto:osmocom-sdr-request@lists.osmocom.org?subject=unsubscribe List-Archive: http://lists.osmocom.org/pipermail/osmocom-sdr/ List-Post: mailto:osmocom-sdr@lists.osmocom.org List-Help: mailto:osmocom-sdr-request@lists.osmocom.org?subject=help List-Subscribe: https://lists.osmocom.org/mailman/listinfo/osmocom-sdr, mailto:osmocom-sdr-request@lists.osmocom.org?subject=subscribe X-List-Received-Date: Wed, 14 Mar 2018 19:57:59 -0000

Hi,

I'm new to SDR and I have a question about the data values that are generated by the rtl_fm program.

I got rtl_fm running on my system (cygwin running on Windows 7). It is sending a stream of 16-bit signed integer values to stdout which I can redirect to a file or pipe to an audio playback system, such as sox. So when I do this:

rtl_fm -f 88.1M -M wbfm -s 240k -r120k -g 30 | play -t raw -r 120k -b 16 -c 1 -e s -V1 -

I hear sweet, sweet music! Hurray!

But how do these 16-bit integers relate to the modulation level of the carrier? I'm assuming that the values are directly proportional to the instantaneous frequency deviation of the transmitted signal. Is that right? If so, how do I determine that ratio? I'm hoping to use this to build a device that shows total modulation (as a percentage of the maximum modulation of +/- 75kHz) for an FM radio station.

Also, I'm assuming that the sample-rate of the output data stream needs to be at least 120kHz because the baseband signal includes the stereo pilot (19kHz), the stereo subcarrier (38kHz) and an RBDS subcarrier (57 kHz).

Any insight would be appreciated. Thanks in advance,

Theodric Young