报错：json read eeror 这个是怎么回事 post json格式不对吗？还是。。。 请教一下  本人新手，请各位大神不要见怪。。

string strbaser64 = Convert.ToBase64String(GetPictureData("D://16k.pcm")); // 图片的base64编码
                String host = url;
                Encoding encoding = Encoding.UTF8;
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(host);
                request.Method = "post";
                request.ContentType = "application/x-www-form-urlencoded";
                request.KeepAlive = true;
                String str = "{ \"format\":\"" + "pcm" + "\",\"rate\":16000, \"dev_pid\":1536,\"channel\":1, \"token\":\"" + token + "\",\"len\":\"" + GetPictureData("D://16k.pcm").Length + "\", \"speech\":\"" + strbaser64 + "\"}";
                byte[] buffer = encoding.GetBytes(str);
                request.ContentLength = buffer.Length;
                request.GetRequestStream().Write(buffer, 0, buffer.Length);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
                string result = reader.ReadToEnd();
                Console.WriteLine(result);